I believe the RailKing is semi-scale. Is that correct? (I own no MTH) I'm sure you're aware of it, and is part of your decision.
I owned a Legacy 0-8-0 (Pere Marquette 11251) and it ran wonderfully under Legacy ..... operation and sounds. I really liked it.
I had another that I sold to a forum member and he ran it constantly in his club's displays at museums.
Good luck.