What class do you intend to run, and what size are you looking for? Also, how much experience do you have autocrossing, and how competitive are you (ie, do you suck or kick everyone's ass...)
The RS3's are generally considered the "tire to have" for larger street tires right now. They hold up to heat very well, but wear considerably quicker than the Dunlop Star Specs. The Dunlops do well when sprayed down after every run on a hot day, and will last a bit longer than the RS3's, all while being very very close in times.
If you're wanting slicks, check out Hankook Ventus Z214 C71 compound. A very good autocross slick that will be 90% as quick as Hoosiers at about 70% or less of the price.
If you haven't been autocrossing long, I would highly recommend sticking with street tires until you get more experience. Learning to drive them correctly will make you go a LOT faster when you switch to slicks, as opposed to a noob switching to slicks and relying on them to fix their poor driving. Plus, switching to slicks when you're overdriving street tires means you'll just burn through the slicks that much quicker than you should.
'02 Mini Cooper #40HS