I'm on a budget but glass is my #1 investment. I sold a bunch of stuff around my house and bought the best optics known to man! Leica 10 x 42 bino/rangefinder combo plus Swaro spotter. I purchased my Leicas for a fraction of the new price via cameraland demos and my Swaro nearly new on the classified section of this website for about 1/2 price. It took time and patience to find great deals but they are a lifetime investment!
I don't need anything else for hunting Colo, Wyo, Utah, Nebraska, Alaska., etc. If I hunted cows deer I'd likely add 15x binos but don't really need them for open country hunting. I can see where 15x would be a nice addition but I'm not going to carry around 2 binos plus my spotter. 15x is way to small of power for field judging. I can find a antler tine, white rump, leg around 1/2 mile away in brush that wouldn't be possible to spot with 10x or even 15x. My 10x are ok for hand holding and there is no way in hell I'd try to hand-hold a hefty pair of 15x all day for multiple days.
Do yourself a favor and buy a bino/rangefinder combo. No need to carry both and it will really speed up how quickly you can range and shoot a critter.