AS5 is supposed to be more expensive but a reseller can charge whatever they like, I suppose.
AS5 does have marginally better thermal transfer, but not enough to really matter, particularly on chips with a heat spreader. If you have an open flipchip, it matters the most but even then not enough to even get up and walk across a room to grab the AS5 if you already had the Ceramique in your hand.
AS5 is more trouble to clean up, for asthetic reasons- after wiping off Ceramique you dont' have the dull grey residue so visible as with AS5. AS5 is slightly capacitive but hopefully you won't put so much on that it'd matter. Ceramique is a handy field-polish for 'sinks, if you don't have the time or supplies on hand to lap a 'sink, vigorously rubbing some Ceramique against the sink will polish it and force some compound/metal mixture into the microscopic valleys of the 'sink base which is good.
It would be easy to simply state "AS5 is better" because of it's marginally better performance, but seldom (actually, never) should a part be that close to overheating that it should matter one way or the other, but by the same token, every tiny little benefit in cooling will additively contribute to lower temps, so if you have several sub-optimal things combined, it might be a few degrees higher temp which is most significant to overclockers.