From my experience, Ceramique's advantages (vs ASIII):
1) slightly easier to apply (its thicker).
2) pure electrical insulator (in case you accidentally get some of it on the mobo or something).
3) very cheap in 22 gram tube.
4) performance maybe a tab better.
All in all, buy Ceramique if you don't have a good compound already. If you have AS3 already, stick with it till you run out.