I would get the CXP33. They are used in cyclocross, because they are extremly stable and they are more aero, which is a factor.
But the most important question ist the quality of the person building the wheels, both rims hold up very well, if the wheel is built correctly. About the hubs, if you have Ultegra hubs, there is no reason to change. If you want to spend some more money, aero spokes are an idea, but only if the person building the wheel is really good. I am quite sure, that a wheel build correctly with aero spokes and mavic CXP33 beats some of the modern carbon wheeels for performance. Not from a pure aero point of view, but if you include stiffness, breaking surface etc. to the mix.
I train about 5000km a year on Ultegra/CXP22 and never had an issue so far. Bought the wheels for 50 USD (used), trued them, and never had to work on them, except for a yearly tire change. Every 10.000 km I would have the hubs opened, cleaned and greased.