The upside/downside decision between a 300 and a 400 is, in my opinion, based more upon space than on anything else. And space can be looked at in two different ways: The space it has inside and the space it takes up.
For us, the space inside is a necessity. As a competition team, we want to be able to hold everything in one location and not have it spread between multiple places, be they Cambros and/or coolers. The 400 will allow us to hold all of our prepped meat prior to cooking and all of our cooked meat prior to turn ins in one box.
The other consideration for us was the physical space it takes up. A 400 is larger than a 300 but not so much to be a burden. However, if we had to worry about taking an extra cooler or another Cambro, well, that would be a problem for us as we travel in a minivan and space is precious.
All in all, the 400 is a single box that works for us in both interior and exterior dimensions.