depends if your a girl or not?
Seriously, the street is a cheaper version of the Speed. Its a fantastic bike, esp when looking at the price. It does everything so effortlessly... I had it on 1 wheel after about 200meters of riding, so it still has the fun factor. The chassis is great too....
But the speedy is just plain angry.... so it brings even bigger smiles.
If your going to commute at all... get the street. (but get the R, as the std Street brakes are not fantastic). If you want to go nuts on the weekend... get the speed. You wont go wrong either way.