I'm going to be unpopular for saying this but I think they are more desirable.
the S4 was a purpose built rally car which was only slightly modified for road use.
The Sport was basically a road car with a bit cut out of the middle to try and get it to be able to compete with the likes of Lancia and Peugeot, and then turned into a rally car.
None ought to be rarer than an other, after all they all had to build 200- but when was the last time you saw a road going S4- I've seen plenty of sports, rs200s and a couple of T16s but never a S4.
The normal ur wouldn't have been able to go rallying without major mods whereas the normal Integrale could have done with little mods (obviously this has a lot to due with the homolagation rules of their respective groups, 200 v 5000)
obviously this is a very simplified view
I'd love both though and hope to compare owning the base models soon
