For me, if I'm seriously considering shooting a deer at 500 yds, I'd take the 12x over the 6x everyday and twice on Sunday. Not sayin' it can't be done at 6x. Just that 12x makes it look like a deer, not a mouse. I'd also find a place to do some serious practice at that range, whatever scope you decide on, before deciding to pull the trigger on live game at that distance.
I've never even laid eyes on a moose before, so I'm no help there. I'd probably still feel better with 12x at that range though.
I just don't understand why people are resistant to higher powered scopes. They do cost and weigh a little more, and may be a little bigger. But to me, if you can deal with that, you get so much more capability with the higher powered optic. And you have the option, in your case, to turn it down to 2x! Doesn't get much lower than that, if you're hunting the thick stuff. It's a win-win with the 2-12 in my opinion.