I vote for the kit.
I built a great plains rifle in the .54 caplock. Its a hell of a lot of fun to shoot, and the fact that you made it makes it so much better. The kit isn't tough to do, you could really assemble it and shoot right out of the box, but with some careful work you can make a hell of a gun out of it.
I can't speak much on conical vs. ball. I've never shot blackpowder besides this gun, and never anything but patch and ball with it. Its a shooter, thats for sure. I'm still learning the ropes and getting a feel for it, but with the standard iron sights I hit a grapefruit from 50 yards the other day. Its more accurate than me and I haven't even tried to work up a load for it yet. I think a .530 ball has plenty of killing power for large game. The ball cuts a groove about 10 feet long and a few inches deep in the ground behind the target I shoot at. Thats only over 60 grains of FFg Goex, the rifle will take 110 or 120.
Also, if you decide you really have to be able to shoot conicals, you can buy a hunter barrel in the white and finish it to match your gun. Its costs about $175 if I remember right. Its got the same hooked breech and everything, they're interchangeable. Then you could have one for hunting, one for plinking switch out whenever you want.