I have to take a contrarian view to the ones being written above - I don't believe the 12x are a winner, and most of the people I've spoken to about them agree. I spend a ton of time with birders, who know more about binos than we hunters ever will (and spend more time behind their glass than us for obvious reasons). While their criteria is a bit different than ours, they are usually spot on.
The 10x50 SV is considered the best glass out there. It gathers more light than most 10x can because of the huge objective lens. It is crazy crisp. Colors are beautiful. But...the rolling ball effect of the EL SV is there. I hate it, it ruins all SV / EL glass for me. I want a flat picture, not one that looks like a distorted, round scroll when I pan.
The 12x have the same issue. Some people can't see it, or aren't bothered. I can't believe that other people can possibly not notice it. Not that I'm right and they're wrong - it's like onions. They taste worst than vomit to me, and for the life of me I can't see why people like them. Onions are my dad's favorite food.
The 15x HD are ergonomically awesome, light and balanced. I like the image through them as much as I like the image through the 12x SV...until I move the bino even a fraction of an inch. Then, the 15x really shines. And the extra reach of the 15x pairs better between an 8 or 10 and a spotter.
Wider FOV is overrated for me. That isn't even an advantage, it's just an automatic benefit of less magnification. I don't want a narrow FOV, but I don't need a wide FOV either. I prefer a small scene, and searching it well, vs a large scene. It's easier to see a trace of a buck under one tree, when that is your entire picture, than one buck under one of fifty trees. And yes, I realize that glassing a smaller area means an increased likelihood of missing movement in the absent periphery. It's a trade I'll make for the increased magnification and a more focused search area.
As far as light transmission goes, it's nearly a wash, with the edge going to the 12x. But, that isn't because of better technology, it's because of physics. 12x50 has a better ratio than 15x56. But again, at a cost. Most feel, and I agree, that the 10x50 hits a sweet spot. The 15x56 is the next sweet spot. The 12x50 doesn't carry either advantage, but is trying to be the best of both worlds. For me, it isn't.
Oh, and the eye relief is generally ill-received on the 12x. But I can't weigh in, because I don't have ER issues.
Check this thread for more info. This site has a mind numbing amount of info.
http://www.birdforum.net/showthread.php?t=313764