Welcome aboard !
Sounds like you will be living and hunting in the eastern part of the state? I lived out by Glendive as a little kid, then spent most of my adolescence in Billings. My wife and I last lived outside of Bozeman for @ 8 years before moving up to Alaska.
I own both of the rifles that you mention, a 527 in .223 and a 550 in .30-06, and have used both for hunting gophers, rabbits, pronghorns, whitetails, mule deer, and elk while in MT. They both work well within their limitations.
If you don't reload, the better bet will probably be the .30-06, simply because of the availability of hunting ammunition. In eastern MT, almost all .223 is going to be in a varmint bullet configuration. These bullets are light weight and frangible, so that they disintegrate after penetrating a few inches. Good for gophers, but bad for antelope on up. The 527 in .223 will handle up a 62 grain bullet, with the best commercial ammunition for hunting purposes being the Federal Premium w/ a 60 gr. Nosler Partition, the Winchester 62 gr. PSP, and the Remington 55 gr Core Loc, in that order.
The 550 is going to be close to 1.5 pounds heavier than the 527, and will be harder to carry all day long if you are hunting the Missouri Breaks and doing a lot of hiking. But that weight will also help steady the rifle when it comes time to shoot it. The .30-06 is a very flexible cartridge, and has the power to thump animals both near and far, and also to give a little more insurance if shot placement is not perfect (the .223 needs perfect shot placement!). Any gas station will have the ammunition, and Remington produces a line called "Managed Recoil" which works very nicely, if you are sensitive to that. A 165 grain bullet will kill anything you mentioned hunting, at distances that sometimes seem staggering.
Keep us posted on what you choose, and best of luck!