Of course, part of your decision should be based on how far you're goinjg to be shooting, but if you're going to be shooting Black Powder, strongly consider the 45-70 over the 45-90. Not that there's anything wrong with the bigger cartridge, but everything has been really well researched and documented for the 45-70, and the information is much more readily available, so you'll be much less likely to reinvent the wheel. In addition, components are more readily available and less expensive for the 45-70.
As far as which gun, that's pretty much a personal preference. I went with a Sharps because I like the appearance, and I wanted a double set trigger. Double set triggers are available on some of the other guns, but they are almost standard on the Sharps reproductions.