Single speed is much easier to work on, and just as capable of top end speed as the two speed. The two speed is can give you more flexibility where you need lower end power - either for riding conditions or for using pipes with little low end.
I've got two Puchs. One is the single speed maxi and the other is a two speed newport. They're both basically stock and top out at 30mph. Frankly, I wouldn't want to go faster than this (I'm a real minority here, i know) without a helmet and without putting a MUCH better set of forks and brakes up front (i'll never forgive puch for putting so little effort up there).
But I spend a lot more time on the newport because it's so much more flexible over a wide range of riding. Short steep sections, trails, loose sand, etc, and the lower gear comes in so handy.