First and foremost, weight is the most important consideration, depending on how fit you are and how far you're trekking. As always.
All things considered, generally speaking, I like my 6.5x21s, or a 6x28 or 7x30 (or similar) for all-timber (no open areas longer than 150 yards or so). Porros have a better depth of field for making out bedded deer, and give better value (quality/price). For mixed terrain, which is most common for me, I like an 8x30. Keeps it light, and is the best all purpose rig that still works. For wide open, I like a 10x - either a 10x32 if I'm going a long ways and need to keep it light, or a 10x42/43 if only going maybe a mile or so from the pickup on foot.

As for which, depends on budget as always. I like the sweet spot for value, which is the $120ish-$400 range. No, they're not as good as high end, but they give you what you need on a budget like mine. Go over $200 if you can. Most anything in this range works pretty well. My 8x30 Brunton porros and 10x32 Zen Ray roofies see most use (forget the models of these, but they're both the lowest end lines - I think Echo and ZRS - also, I do believe Zen-Ray is now kaput - shame). I've used the foreign-made Loopies, Pentaxes, Eagles, and Nikons as well in the past. Budget determines brand and model, and thus glass grind quality. You can go as high as you want. As always, check ebay for used first - better deals. The choices are myriad, so do your research.

I didn't go into detail about the exit pupil size, relative to weight and size, but those are all weighed into my final conclusions above.

Generally speaking, for birding, and 8x42/43 is the sweet spot. For hunting, you might want lighter depending on how far you go in, and you might want more power, depending on how wide open and how good your eyes are. Remember, you always sacrifice exit pupil size, field of view, and practical stability with more power, ceteris paribus.

Now if you *can* afford that $500-$2000 range, keep in mind that the extra glass grind quality will allow more power to be used with good clarity. So if you're in the higher end of things, and can't decide between a 7 and 8, or 8 and 10, round up - but you do still sacrifice the exit pupil size, which makes it harder / slower to find your target.

I guess you might could say that under 40 years old, an 8x42/43 (for woods) or 10x42/43 (for mixed or open) is best for under 40-45 years old, but over that, and 8x30/32 (woods) or 10x30/32 is the the best balance as you get older - just lighter. Lighter is critically important for us old dudes. As for roofies vs porros, that's mostly budget-determined. Both work great. If you're going for the value sweet spot of $120 - $400 but can get more in the $250 to $400 range, then I'd go for a roofie, as they as a bit smaller. If on the lower end of your general value category, the go for a porro - an ounce or two heavier, but better glass for the given price.

One more more thing, if your hunt is the spot-and-stalk style (typically sheep, goats, mulies), AND you have the wherewithal (youth and strength) to pack in more glass AND a tripod to use it without shaking, then it's helpful to have more glass obviously, to determine whether it's worth it to try to close the distance: either a compact spotting scope (maybe a 15x50 or 15x60/65), or big binos of higher quality in the range of 10x50, 12x50, 12x56, or possible even bigger, such as 15x56 or 15x70ish.