nothing!
They are both good cars, as are most on the market....buy what you have support for at your track, period end of story......get your tires dialed in and start practicing! Get a car, stick with it!
Personally, I would go with a used rb5 sp2/wc for your price range
On a closing note, I have a buddy that runs a Special K RB6 in stock buggy and his car is plum stuck and stupid fast