The 38 mm rims are heavier but stronger than the 24 (?) mm ones. Mostly, it was the strength that attracted me to these. I'm using them as every (dry) day wheels, and didn't want anything too deep, to not be too susceptible to cross winds. Also, it was light weight I was after with these, which also leads me to shallower rims - and the 38s were the ones that were strong enough to hold me.
The truth is, I've had a hard time figuring the aero thing out. I could babble some more, but I'll just say I think these are slightly more aerodynamic than my old (stock) wheels, but not terribly more so, more aero than MadFibers, and less so than Zipps.
At the end of the day, I'm happy with them. Also, these are tubulars - I'm enjoying that. They feel great through corners!