If you had asked on the class C forum, you'd probably get everybody saying how class C's are superior. In matter of fact either one is suitable for many circumstances and conditions. Most class C's are designed and laid out more for family use, with room for children and such, while most class A's seem to be intended more for a couple alone.
A class C is perhaps safer in a crash, with a stronger cab structure and front airbags and such. Basically no motorhome is great in crashworthiness, aside from the innate advantages of having a much higher mass than most other vehicles you may tangle with.
Class C's may also be a bit easier to service, or at least to find places to service, due to the greater commonality with regular vans. Most class A's don't have a door to the cab, which can be a big convenience for fueling up and such like. (It also means you're not locked out of your unit should one door lock jam up.)
None of the advantages or disadvantages are deal-breakers in my estimation. Much more important is whether a particular unit meets your needs and wants and uses.