Trapdoors have one big inconvenience: you cannot clean from the breech. If you shoot a lot that will be a BIG issue.
Well, actually you can if you use Moosemilk and a Bore Snake. The Snake does a great job, place the muzzle on your boot or a towel on the ground, drop the weighted end through the breech and out the muzzle, spray some Moosemilk in the breech then hook the weighted end on your tailgate or a bench and pull the Bore Snake through and out the muzzle. One pull through and it'll be as clean as 4 or 5 runs with several patches and rod from the front! That is all I used on the museum quality original I had and later sold. The collector/Shooter that bought it said the bore was in near pristine condition and that the Bore Snake had helped keep it that way as it leaves no marks going in or coming out. Your mileage will vary, of course, but a Bore Snake works well on most all firearms, I use it on my Pedersoli Sharps with Moosemilk (1 part Ballistol to 7 parts water in a squeeze or spray bottle) on the range and only a bore-guided rod and patches for final put away cleaning before storage. The alternative is to use a good and solid cleaning rack, drop the weighted end through and out the muzzle then place the rifle upside down in the rack, spray Moosemilk on the Snake and pull through. You can get the same effect with a rod and patches from the front/muzzle, thusly cleaning the fouling out and dropping the patches down and away from the breech, this works well on all types of 45-70's and generally keeps the gunk out of and away from the action.
Regarding the choice of 45-70's, either is good, if you decide to start with a 'new' Trapdoor model and then move to a Sharps later, you'll recoup most all of your investment. If you go the 'used' route buy the best you can afford.
You didn't mention what part of the country you are located in, it might help us steer you in a better direction.
Best regards and good shooting!
'Ol Gabe