Things to consider that may make the decision for you...
The SDT has been long gone "new from dealer" so the going price is over $900 (unless you are lucky).
It has the 16" barrel and straight stock that is VERY desirable to many wiht only 500 available.
The STP has the same desire that the SDT has in that it is a 16" barrel but pleases the pistol grip crowd. Not sure if still available "new from dealer" but if not expect to pay over $800.
The SDG in my opinion is the least desirable because it is basically a 1895gs with Laminate and Pistol Grip.
The price for this pakage does not seem to make sense when there are so.... many 1895gs rifles out there usually can be had for $500 to $550 GENTLY used. (you need to be patient). Then if the pistol grip is what you desire there will be people knoking at your door to do the swap for that straight grip.
I really couldn't care less about the pistol or straight grip. The STP was what I was able to get when they were available and I can't find a thing wrong with it. It shoots dead accurate, allows me to handle the recoil well, and is quick to aim in close quarters.
If I were to be in the market for one of those.....and I'm not.......I already have the ultimate .45/70 (1895GS)..... it would be the SDT..... I like straight stock rifles myself.....and I'm pretty sure the SDT was made by Marlin in CT........not sure about the other 2 models......I don't keep a good track record of the non catalogued Marlins.
I know I am probably in the minority. But SDG is fine with me! SDG (shoots darn good)!
I picked this one up for $575 LNIB- FTF no FFL or tax with a box of shells thrown in for $575.
Action was a little stiff at first, but with lots of fondling, she has loosened up quite a bit!