I recommend the carbine over the buffalo. The carbine is lighter and more elegant. Also, the buffalo model sold in the USA comes with a brass frame or a stainless steel frame. You won't want to go putting 40 gr of black powder in the brass frame one, uses 20-24 grains at the most if you don't want to damage the frame. The plus with the brass frame buffalo and shoulder stock is the looks of it. It is pretty! And the shoulder stock for the 1858 buffalo will also fit the 1851 colt (at least my stock does anyway).
As I see it if I get the attachable stock I can attach it to other Remington copies (shorter barrels) for additional carbine looks...
To attach a shoulder stock, any gun will have to have the notch cutout on the metal bottom of the grip frame. Not all colt and remington clones have the cutout. The 1858 buffalo models do.