I wanted a fast boat after a bunch of kayaks/surfskis blazed by me in my first kayak race back in May 2008 (in Necky Manitou 13).
Spent some time in a handful of boats, Epic 18x, Think Evo, Think Fit, Epic V10S, and raced folks and looked over (but didn't paddle) Mohicans and Thunderbolts.
I ended up with V10S, and had a general liking towards the surfskis. Why?
Cost
It seems in general surfskis have more lower-end build options which are way cheaper and comparable builds seem to be at least a few hundred dollars cheaper than unlimited kayaks. I wanted a model with a fiberglass layer to help with any hard knock issues the boat might receive.
Exposed Cockpit
It is nice being able to throw feet off the side to gain stability or to go for a swim easily. Motorcycle versus car feeling perhaps? No need for pump/paddle float/spray skirt. Was also nice to know that as I work on my forward stroke I don't have any restrictions in terms of working on rotation, and that all surfskis (if I ever decide to move up) have basically the same type of cockpit: a molded seat and foot pedals.
As far as speed goes, don't think there is a much of a difference, at least on flatwater. As conditions "improve" the bigger volume bows of the surfskis come into play and I have a feeling would be faster.
I've had the surfski for over a year now, and other than recovering from sticker shock of the boat and wing paddle, have had a great time going out on day trips with local clubs (waterproof bag in front of pedals for clothing/lunch) as well as plenty of flatwater and a couple of ocean races.