Of course SCIV.
I agree with Bloodreign that DXX Had a "groovy" soundtrack -- actually worthy of the original ROB redbook stuff,...
But DXX had stiff controls and perhaps too skewed a difficulty (probably why BR likes it). THe art style was neat though, and it definitely put the SNES to good use in some stages.
But yeah, I'd definitely play ROB or SCV IV before DXX.