May be too late .. but here goes anyway. I have hunted eastern Montana several times now. And that is about as open a country as one can imagine. A good quality 8x40 or 10x40 is the way to go.

A 15x50 ? Not for me. If you do a bunch of stalking a bino that large and heavy will start feeling like a mill stone around your neck. And the 15X .... going to be tough to keep steady enough to stay on "spot" at long distances. And that is a big deal when you are deciding to go or not to go. If you decide to go with the 15x50's and they will attach to one, I strongly suggest a tripod.

I tried toting a spotting scope ... only once. May as well have brought a baseball bat for all the good that spotting scope did me. The binos did just fine for the 2000 +/-yards I was spotting across.

Also I started out in MOntana with relatively inexpensive Nikon 10x40's. About $200 range price wise. Fine for down here in SE Alabama, but not nearly crisp enough to make out details at 1000-1500 yards.

Vortex may make good stuff ... I don't know. Pentax's binos that are in the $500-$600 range are a pretty darned good value. I use a Swarovski 8x40. Took me a while to save up enough to buy them but worth every dollar when it comes to long hours spent spotting across 1-3 miles of bad lands. Especially when daylight is at its weakest.

And as for which is more important? If I am forced to choose, I'm top dollar into the bino. That is what you'll use to find the game. A good quality scope can usually get the job done when it comes time for the kill shot.