I would get the Viper HD 15x. I have used them for several years with excellent success. If you don't do a lot of backpack hunting, I would keep the Nikon's for around your neck and the Vipers for in your pack. The 15x Vipers are a tad on the heavy side, so I don't recommend you wear them around your neck, and when you glass with them, you will want a decent tripod and adapter to see their full potential.
I can glass for hours at a time with my Viper 15s, and my personal record for spotting game with the Vipers was a bull elk and four cows at 6 miles. I would pass on the 12x if you are going to be glassing longer distances and opt for the 15s instead. However, if you don't want to carry two pairs of binos one in the pack, and one around your neck) like I do, the 12s might be a better all-around option. I sure love the extra range I get with the 15s, and I don't mind the extra weight because they save me an aweful lot of walking as I can see so far with them.
You will be happy with either one, and for that kind of money you get a really outstanding bino.