What's the twist rate of the barrels? Some of the 50's, have twist rate that's not good for a ball, better suited for sabots. For ball ammo, look for a 1/45 or higher ( bigger the second number the better for a ball). Less than 1/45, better for sabot. I shot a 50 cal.. Here in Georgia, we don't get "big" big game. If your hunting out west, Mule Deer, Elk, etc... you may want to go to .54 cal.
Makers........ Price call tell you all you need. The cheaper the rifle, well.... You didn't mention if your looking at traditional or in line (modern). The cheaper traditional rifle are pretty much you get what you pay for. I have a Traditions Kentucky, it shoots great but it's not something I feel will last a long time. I also have a new purchase (used) Hawken (build in the late 70's to 80's), it's build like a tank and about as heavy.