The conventional wisdom amongst (some but not all) target shooters is that the .45 x 2 4/10 (.45-90) will show an advantage over .45 x 2 1/10 (.45-70) only at ranges greater than 800 yards.
Differences in terminal performance on deer? Forget about it.
Get the .45-70. There is so much more available in terms of guns, components, data, and 132 years of experience for it. The .45-90 is not much harder to load for but it cuts your choices and increases your costs.
I don't know your budget but suggest you START looking at Pedersoli copies of '74 Sharps. I have heard from Cowboy shooters of decent guns from some of the other Eyetalian brands but I have seen some that were ok and some that were junk; it is a crapshoot. I do not see them at all on the BPCR Silhouette and target ranges where score is kept and excuses are not.
Shiloh and C. Sharps are much nicer if you are willing to pay for quality.
You can start learning about black powder loading at:
http://www.ssbpcrc.co.uk/Resources/Introduction to BPCR Loading.pdf
There was an article on less elaborate .45-70 loading in the April issue of Handloader magazine. I don't understand his thinking, he talked about loading BPCR with less expensive bullet molds and other cost and time saving measures... for $2000 rifles. He got good enough accuracy for hundred yard deer hunting, but very little that would do for target shooting at any great distance.
The SPG Black Powder Cartridge Handloading Primer is a good reference, as is Mike Venturino's book 'Shooting the Buffalo Rifles.'
Lots of good information at
http://www.shilohrifle.com/forums/
and your Sharps doesn't HAVE to be a Shiloh to learn a lot.
A Buffalo Arms catalog will show you everything you need except the powder itself.
http://www.buffaloarms.com
I am all in favor of real black powder and use nothing else in my Higwall. The fakes are easier to get if you only shop at Walmart but they have peculiarities of their own and they just don't SMELL right. If you just must, beware of Pyrodex. Although it does not accumulate fouling during shooting, it is harder to clean and more corrosive if you miss a speck. As a friend found out on a Marlin repeater shot at CAS.