There are a couple of aspects here - first is viewing conditions, which no one can control and which no binocular can meaningfully resolve (I remember at the last Birdfair I attended there was so much heat haze that it didn't matter whether you were looking through a 10x42 NL or something much lower down in price, the seeing was awful).
The other is characteristics of binoculars that make them suited for observing at particular distances - in which I find some real differences. I agree with this post from one of the members here (
link):
It's a combination of optical characteristics (exit pupil size, which affects eye comfort) and handling/ergonomics (better balance, steadier grip allows tiny objects at long distance to wobble less and be seen more easily - IS and tripod-mounted binoculars are the ultimate expression of this).
so for birding at short distances (sub 50m) where you are often spotting the birds by eye and need only a fairly quick look to discern field marks, I find the modern fast focusing 8x32 or 8x42 birding binocular (Conquest HD, FL) works very well. For scanning further afield I prefer something I can hold as steadily as possible, larger exit pupil and slower focusing helps, and for really significant distances higher mag (10x).
I've visited (albeit only briefly) the SF Bay Area where, from what I gather, the OP lives, so to attempt to use local examples - if I were walking around Golden Gate Park, just general birding, I'd use his 8x42 Conquest HD (ought to work great for that situation). If up on the Spencer Battery trying to follow peregrines off the top of the Golden Gate Bridge 300-odd metres away (and which could fly a lot further), I'd use his 10x50 Maven from a sitting position, elbows braced on my knees - but if wind was forecast to be strong that day would go back to the 8x42 to minimize wind buffet. Of course, in practice, it isn't necessary to have the perfect binocular for every situation - I've brought my 10x40 Dialyt to California and many other places, used it in a wide range of birding situations, and truth be told, have had very little reason to complain.