I've been loading for, and shooting, the .45-120 Sharps for about 20 years, but I only shoot smokeless powder in it. I really, really don't like the mess and smell of blackpowder. (And I don't care to hear from blackpowder shooters how they love it) My bullet is an RCBS 405 grain RNFP that falls out of the mold at 420 grains. This is a gaschecked bullet, though. I've driven it as fast as 2,100 fps through my rifle, with no leading. There is a caveat, though. I use Super Grex as a case filler, as described in Handloader Magazine. I can't recommend it for anyone else, but it works for me.
My powder of choice is IMR 4895, in large doses. This rifle is a thumper, and not for the timid. Most people won't shoot it more than a couple of times before handing it back, though I once had a female game warden that weighed about 140 pounds put 21 rounds through it in one session. She loved it and only stopped because that was all the ammunition I had with me at the time.
Since you haven't bought the rifle yet, I would suggest that you go with the .45-90 caliber instead. That is, unless you want the blackpowder loads for the extra power. When using smokeless powder, the .45-90 is more efficient and will give you the same velocities as the longer .45-120 case. It's only with that smelly, dirty blackpowder that there will be any advantage to the longer 3 1/4" long case. (Did I mention I don't care for blackpowder?)
Another factor is the availability of brass, and the cost comparison between the two calibers. Starline is making .45-90 brass, as are several other makers. The only current manufacturer of .45 Basic brass that I'm aware of is Jamison International, who bought all the BELL brass making equipment from PMC (and then bought PMC). I don't consider Bertram's products as reloadable brass, since I've had so many problems with it over the years. It cost about the same as Jamison's product, so if you decide to go with this caliber, buy the good stuff and don't waste your money on the inferior Bertram product, unless you like cases with splits right out of the box, before loading, and primer pockets that the web blows out with the first loading.
If you decide to buy the .45-120, you will draw onlookers when you take it to the range. I like mine, but there are the things I've listed above to consider.
Hope this helps.
Fred
PS: Did I mention I don't like blackpowder??