I'm a current StudioDeck owner who spent a day with an UltraDeck in one of my systems - the one with the Decware tube amp shown in my signature.
The UltraDeck definitely has more isolation with more to the plinth and a bigger platter. I've acoustically always found Delrin to be quite dry, on the darker side, and dampened, so for me, the extra isolation wasn't something I found necessary. The Cardas wiring in the UD, that's for sure a plus. Same length arm, but the UD has improved bearings and material composition. It did not strike me as a night and day difference from the SD, but again, only spent a day with it.
From a tactile standpoint, the UD definitely has a bit of a more robust feel in your hands, but it doesn't by any means make the SD feel cheap in any way. Motor is the same in both, if I'm not mistaken, and a hell of a good one at that. No speed controller and it is bang on to my ear.
Though they came into play at different times, ultimately I own and am very happy with my StudioDeck, and never have a situation where I say, gee you know the UltraDeck would really shine on X record in comparison to the SD, so I think that may hopefully answer your question/tame the tiger. I think
@Static Shock is right in not to get pulled into the rabbit hole too far! The MoFi decks have a distinct voice, and it is firmly on display in both decks.