the data from zipp actually shows with a 23mm tire, the 404 is only 2 seconds faster per 40k in terms of aerodynamics. So, you get almost all the benefits aerodynamically, while being more sellable to road cyclists. They are close wheels in performance though.
I agree, for most riders it just makes more sense to go with the 404 as it is faster but it's not by much. If you're a roadie and you have some serious skepticism about handling in winds or y actually think weight matters, the 303 is an option.
I thought the 303 was built with a tough (Kevlar) rim bed and a rim shape that was better for wider tires. Also some sort of compliance (carbon bridge tech) so if you bottom out the rim deforms to avoid cracking? Most of our interesting road races on the front range feature bumpy dirt road sections (Boulder Roubaix, Kopenburg) and 25's or greater are nice. Plus I use mine for cross wheels in the fall/winter and swap road tires for the spring classics. I keep a 25 mm tire on for all of the events, and switch the front 25mm out for a 23 once the dirt road races end, but I'm a dork.
For triathlon, I would stick with the 404/404 combo. I had 303, 404. Got rid of the 303s, the 404s just felt like I had a better "sail effect" from the depth on windy courses, yet could handle the crosswinds just fine. Including the Hawi crosswinds in Kona. I've used the 303's there and the 404's. Definitely think the 404's were better overall for versatility. Although if you can get a hold of a disc, use that often, or an 808 for the rear. Unless you go with a different brand, you may be able to save some $$ over Zipp wheels.