None of the above: definitely the smart cover.
I can't say I've noticed any difference in speed, and I positively prefer the design of the iPad 1. With the curved edge of the iPad 2, the power and volume buttons stick out, and it's hard to plug in the power charger without lifting the unit.
The smart cover is great. Love the automated on/off, and despite only a small change in actual thickness, it somehow just feels more portable than the iPad 2 in the Apple case.