For city riding, you might as well get a "50cc" scooter - I'd reccomend a honda met, or a yamaha (they've got one with a "trunk" under the seat that holds 9 cubic feet!). if you're interested in acceleration, the 2 strokes are a bit quicker, but they're loud and fart exhaust clouds - stick with a 4 stroke.

As an aside, why not get a crappy Huffy MTB or something and stick a motor kit on it? Get a 40cc motor, do 35 mph and still be able to bike around when not using the motor.