You're looking at a pretty big commitment, as you're probably aware. Truck and trailer, feed, tack, shoeing, worming & misc vet expenses and the rest rack up quickly. They also need to be used throughout the year to be decent during hunting season. You can't just turn them out and expect them to be ready to go. Works best if you have enough land for grazing that's big enough to ride out your door. If you have to trailer in order to ride, you're probably gonna have some fat worthless critters that aren't going to do you much good. Mules are great for packing and can be good for riding but it's hard to find a good riding mule and they usually have one slow gear and a grudging trot, so don't expect speed. You'll have to monitor a horse more closely to make sure it doesn't founder, though, unless you're using it a lot.
I'd say the individual animal and how much you use it is going to be more critical than horse v. mule. A couple horses and a mule or two is usually a good combo - you have an extra saddle horse if someone else goes and you can pack half an elk over the saddles in a pinch, or head back and pick up the mules for a tough extraction. After they've bonded you can generally turn the mule loose in camp and it won't stray too far from its horse buddies. I'm a fan of the portable electric fence for camp pasture, cause you're not up three times a the night unwrapping picket ropes and grinding whatever meadow into a bunch of circular mud holes.
I worked for uncles who outfitted some really steep country (in MT) and horses did just fine, although mules are certainly tougher when it comes to packing. It's more a matter of making sure they are in shape and have enough time under their belt to become "mountain horses" versus "riding arena horses" that balk at every stream crossing and buck when you get near 'em with anything bloody. Your best bet is usually to put the feelers out with some horsebacker friends and pick them up that way. Winter is a great time to pick one up because people are looking to dodge feeding for the season. Wait until hay is really expensive and you'll have more options.
I live in town, so although I miss having horses, I sure haven't been tempted to pick up a few. The elk tastes sweeter with a sweat marinade.