The ED IIs have arrived, and I've been spending a bit of time comparing them with the Sierras, looking at things out the window. Here are my initial impressions...
In the centre of the field, I was surprised to find that the Sierras resolve fine detail more clearly. There's an electricity substation about 40m away and the small writing on the warning signs was consistently clearer with the Sierras. It's not a big difference, but it is certainly noticeable. With textures (like harling on a wall about 50m away), I got the impression that the ED II did better.
The ED IIs seem to have more depth of field. For objects at around 80 to 200m, it seemed as though tiny movements of the focus wheel made less difference in sharpness when moving between nearer and further objects. With the Sierras, I needed to adjust the focus more to get objects at a different distance that bit sharper. Maybe the aforementioned harling (which was at an oblique angle) looked better in the ED IIs because of the depth of field - less need to hunt compared to the Sierras?
In low light (now darker after writing most of this), I'd say there is no real difference between the two bins - they seem to be pretty equal in terms of light amplifying power.
With regard to the other optical qualities, the ED II seems to be better. When apparent, there was very little evidence of CA in the centre of the field, although there was some at times towards the edges. For the same subject and lighting, the Sierras had quite noticeable CA in the centre. (They don't always - they are usually pretty good for this, but it can pop up when things are contrasty, and it's rather distracting when it does.)
The ED II also had better contrast and maybe a little better colour saturation. I note that some have said that they are slightly warm, so the latter could be emphasis of certain colours, but the general impression was that the picture was a little richer compared to the Sierras - I've sometimes felt the latter look a little washed out in daylight (although less so when looking at things that are close).
Looking at vertical and horizontal lines on buildings, the ED IIs are superb with regard to distortion - maybe a tiny bit of pincushion at the extreme edges of the field, but otherwise they're spot on. (Speaking as a photographer with a strong liking for rectilinear wide angle lenses, I'm impressed with this.) The Sierras start to introduce pincushion at about 50% from the centre, and it comes in quite quickly, after which it doesn't get too much worse. Panning around a little, I noticed the rolling ball effect, but it didn't bother me. I've notice something similar with the Sierras with the pincushion distortion - lines of trees, for example, wobble at the edges of the field when panning (the inverse of rolling ball, I guess).
The sharpness across the field in the ED IIs was quite evident as well in the short time I looked at that. The Sierras need to be refocused to sharpen the edges, and still aren't as sharp compared to the centre.
The FoV was effectively the same - I didn't notice any difference when swapping between the two bins. Even when looking for it, it was hard to hold them still long enough to get an idea of how much difference there actually is.
With regard to build quality and general handling, the ED IIs are definitely a cut above the Sierras. They feel noticeably heavier (or, rather, the Sierras feel lighter after using the ED IIs). Weights are 800g for the ED IIs and 700g for the Sierras. The ED IIs have a general feeling of being better put together - more solid feeling, the focus wheel has a bit more drag (maybe a touch light on the Sierras), and the dioptre adjustment feels very nice - positive click to lock/unlock, and certainly easier to turn than the Sierra's non-locking version.
I think I like the open hinge design of the ED IIs. They feel less muckle and are much easier to hold securely with one hand. The fast focus of the ED II will take some getting used to. I quite liked the rapid change in focus distance, but I think I need some time to get used to the fine adjustment once in the zone.
The ED II eyecups turn easily and seem to stay in position okay, while the Sierra cups differ from each other - the left one turns fine, but the right is a tad stiff (which a recent application of some Teflon grease to the helicoids didn't cure). Both are fine for adjusting IPD - nice amount of drag, and no tendency to move out of adjustment. The objectives on the ED IIs are more recessed than on the Sierra, which I think is a good thing, and the caps are a fairly soft rubber and feel better than the soft plastic (but stiffer) ones on the Sierras. In practice, the caps on both seem fine - they stay on and are easy enough to fit and remove, but I think I prefer the rubber ones.
The ED II case is better made and has better padding. It feels like there is some fairly firm closed-cell foam inside, and it's thicker than whatever is inside the Sierra case. The zip closure is also better than the Velcro flap on the Sierra case (which is also starting to lose its grip).
The strap arrangement on the ED IIs is naff. I fail to see the logic in either having the strap on the case or on the bins, and I wouldn't trust the plastic clips as far as I could throw them. As far as I'm concerned, that's an unnecessary point of potential failure for the sake of some bizarre strap-swapping idea that makes no sense. My daily use of bins is as follows - get bins, in case, out of manbag and sling them over my head cross-body style. Go to lunch at works canteen, then head out to the nearby golf course. Bins come out as I'm walking down the car park and go round the neck, case stays slung across the body, hanging out of the way at the small of my back. With the ED IIs, where am I supposed put the case? Am I expected to do this stupid strap swapping nonsense while walking down the car park, all the while having no fear that my pricey bins aren't going to get dropped? It's a stupid idea, and Vanguard need to ditch it. I can't comment on the Vanguard strap because it hasn't been taken out of the wrapper and never will be.
Overall, the ED IIs seem to be very good, although I have to say that I'm a bit nonplussed with the seemingly better sharpness in the centre on the Sierras. I really didn't expect that and find myself wondering if there is something wrong with the ED IIs, or maybe I got a particularly well made/ground/set up pair of Sierras. The fast focus is certainly different from what I'm used to, and I'm looking forward to trying that in the field, as well as seeing how the apparent difference in sharpness fares when looking at birds rather than something that's one step short of a test chart. In all other optical and build quality respects, the ED IIs are better than the Sierras, but I'd have to say that some of the differences are more subtle than I was expecting (like the contrast and saturation - better, but not hugely so).
If anything, I think I'm more surprised at how good the Sierras are, especially for the money (I paid £74 just over 2 years ago). They certainly have their shortcomings (pincushion and soft outer field, especially), but are very much cheap and cheerful, with a strong emphasis on the latter. For something that's worked out to £3 a month if I stop using them now, they've been a bargain and have certainly helped me to take more of an interest in watching birds (even if I don't see myself as a serious birder).
It has also been interesting to see how little difference there is between those and what might be considered amongst the best of the Chinese made bins. Unless there is something amiss with the sharpness aspect, it's mostly quite subtle rather than night and day. Big difference with the CA and pincushion, but the contrast and saturation less so. The usefulness or otherwise of the sharpness across the field will no doubt become apparent when I use them for watching actual birds, as will the much faster focus.