my understanding is that Us set 2 is the rom dump from original 4 board set, and that US set 1 is from the second 2 board rom dump
the 2 board set is the one used by TG for High Scores, so you should be using the US set 1 rom in mame to practice
I have a dedicaded DK (2 board set) and I've played the one at funstop many times (the sprites wern't functioning on it the last time I was there about a montha ago though

) and find the US set 1 version in MAME to be identical and Ive gotten up to around 300,000 on all three machines
I do find set 2 to be a little easier too, but I don't play that one much