Yes, but don't forget the original question was about the 6x30 and 8x30 YF
Unfortunately there's no 8x42 YF (let alone a 7x35 YF, that would be a dream come true). As you say, preferences are, well... personal.
Back to the original question.
I happen to have both the 8x30 and 6x30 YF II by my side (it is one of my favourite binoculars for the combination of performance, price and form factor) and have just retested them side by side not to go by memory.
View attachment 1507783
A couple of considerations I think are worth mentioning in the light of some of the comments above.
Field of view. Yes, the FOV of the 6x30 is (just slightly, half a degree) bigger... but remember it's just a 6x. Doing the "easy" AFOV calculation, the 6x30 ends up with a pretty humble 48º (too narrow for many people, by the comments here on BF), while the 8x30 goes to a respectable 60º AFOV, which is something that matters quite a lot while using the binos, to avoid having the feeling of looking through a keyhole. So, in my personal experience the 8x30 is more comfortable to use (especially if we are talking about using it for birding, as the OP says).
Brightness. There is a mantra held by many: bigger exit pupil and bigger objectives are brighter... but during daylight hours (actually, even during sunset) it is the performance of the coatings/glass that make a pair of binoculars brighter, not the diameter of the objective. For example, I have a pretty nice 7x50 Vixen Ultima Porro (with a huge 7,1 mm exit pupil), it is a nice astro glass made in Japan, but my 8x32 Swarovski EL SV (with a humble 4 mm) are noticeably brighter, way brighter. Only more than half an hour past sunset is the Ultima brighter (which is probably 1 % of my birding time. So an 8x32 gives me all that I want. Funny enough, in my case an 8x42 has no place. I use an 8x32/12x36 during daytime hours and should I want to venture in dark conditions, I use a dedicated 8.5x52 or an AK 8x56 which is brighter than a 8x42 should the conditions arrive).
Coming back to the YF, the above proves to be true once again. The 6x30 is not brighter than the 8x30. As a matter of fact, as was to be expected, the image of the 6x30 has way more depth of field (something I love, I'm a big fan of lower powers, my fav is 7x42 or 7x35), and the level of contrast is also superior, the image is really pleasing. However, surprisingly, the "lesser" contrast in the 8x30 combined with the much bigger AFOV and magnification makes for a superior "apparent brightness" (for lack of a better term). Images appear to have slightly more light on the 8x30, while they are more contrasty and "engaging" on the 6x30, as is typical of low-mag (and the reason I love them). So, brightness doesn't seem to play an important role in this case.
Viewing comfort: another repeated mantra is that bigger exit pupil will be "easier" (as in "more comfortable"), but this is not always the case. I've found that several binoculars with a nice and wide 6 mm exit pupil are less comfortable than binoculars with a smaller one. For example, the aforementioned 8.5x52 Minox HG (it was Minox top of the range back in the day) has a 6,1 mm exit pupil, but the combination of eye relief and eyecup design makes for a less than stellar viewing comfort in my experience. I also remember a 7x42 Optolyth Alpina NG which was a bit of a pain, even the 8x42 Zeiss Conquest HD (a terrific binocular otherwise) is prone to black outs despite having a 5,1 mm exit pupil as I experience during the couple of years I had it (and has been reported many times on many forums). On the other hand, a "humble" 8x30 (with a 3,75 mm exit pupil) like the Nikon EII is praised endlessly for its ease of view. As
@Paultricounty said, there's more to comfort than exit pupil size and objective diameter. So there is a need to study each case, each binocular in this case.
Back to the YF, I personally find the 8x30 is easier to use than the 6x30. I get a more immersive view with less chances of blackouts (despite having a narrower exit pupil). Without having any technical knowledge about optics, my guess is that by altering the magnification and keeping many other things equal, the behaviour changes quite a lot.
As a wrap up, back to my original post. Both are amazing binoculars for the price and size, there is something really engaging about the form factor (although they might feel a bit toyish if you have big hands). If I could only afford little more than 100 €/$, I'd live happily with a pair of Kowa YF (or any of its siblings), and my personal choice as an all-rounder birding binocular would be the 8x30, going along with what
@edwincjones says. I'd only choose the 6x30 over the 8x30 if I already had one (or several) 8x and needed a small and inexpensive waterproof porro, or if I had shaky hands or a limited experience using binoculars (where the stable view and lack of constant refocusing is a great plus). As a true testimonial: both my mother and my daughter have a 6x30 YF as their only binoculars (the first version for my mum, the II for my daughter), while the 8x30 YF II lives in my glovebox.
