I'm working up the cash for a Marshall TSL100 stack. But I can't decide on the cab, I was thinking 2x12 because I've heard the four speaker versions aren't as good for clarity. What do you guys recomend?

Also, what should I get first, the cab or the head?

I'm thinking head first because I have noticed people running heads through old combos. Although, why I'd want to do that...
People say Vader 2x12's are good cabs. I haven't personally tried one though so I can't really say, that's just what I've read.
As far as a good "bang for the buck" goes, it's hard to beat Avatar cabinets. I've had two (a 112 and a 212) and both held their own against more expensive gear. For the less budget conscious, excellent 212's are available from Genz Benz, THD, Soldano, EarCandy, etc.
ESP LTD EC-256 and a Fender Deluxe VM