I'd suggest trying NullDC, but I only just started researching it yesterday so I'm no expert. I've found that the Dreamcast version of Gauntlet Legends is a mix of both Gauntlet Legends and Dark Legacy.
I've just finished my medium res cabinet with original Gauntlet Legends hardware, and am already starting to think about selling the PCB set and running MAME/Dreamcast. Whatever I do, I want to make sure I can still play Gauntlet with 4 players at full speed.
I'd like to know what you decide, and how it works out for you.