If the WDW version of space mountain was as smooth as DL I would agree ... but its not, so IMO, the nod goes to DL. The WDW version is almost Matterhorn like rough.
I think the in-car soundtrack is the best part of the DL version of Space Mountain. With apologies to the Dick Dale fans out there, I love the song itself, I think it goes perfectly with the ride itself and even elevates it to a new level. I generally think the side-by-side seating is better than the Matterhorn-style single car.