I've been a fan since the first game -- I've played Scribblenauts and Super Scribblenauts on DS.
Scribblenauts has a lot of action levels (trying to dig around something, and/or avoid explosives, and/or melt ice). Unfortunately, it didn't have dpad controls, so the physics and controls can be frustrating unless you're super patient.
Super Scribblenauts has dpad controls AND adjectives, and may focus more on puzzle than action. I feel like Scribblenauts Unlimited focuses a great deal on puzzles, and presumably because of the engine changes some things are simpler than before (no ice/terrain tile-based digging, no fancy time machine taking you to a jurassic level, if I recall correctly).
I'd say if you have a DS and you really love Scribblenauts Unlimited, Super Scribblenauts is worth getting, but you might want to hold off on the original game unless you're just totally in love and don't mind potential control/physics frustration.