The best way to pick a tank (if you have the luxury) is to decide what you want in the tank. The worst thing is to say you want X tank and then after you get it set up and buy equipment for that tank, you realize you want a more shallow tank, a longer, deeper one etc etc.
If you love tangs, go 6'. If you want an sps tank, get one that's wider with lower rock scape etc. Go big or go home is a good statement in my mind cause if you buy a 29g it only takes one polyp of peachy palys and in 4 months you have no more room!