If you're buying new, go for the Columbia with the Mercedes 450 and Meritor Freedom IMO. If you ever plan to drive in traffic you won't regret it. As far as the box goes, from my research they stack up like this:
Showhauler: Most flexible design, highest cost
Renegade: Somewhat flexible design, lower cost
Haulmark: Totally inflexible, lowest cost.
All three use different construction methods for the shell. Check them all out and go with what suits you best.