Probably Flamethrower. Assuming you have Dragon Claw or similar, there is almost no situation in which even a super effective Rock Tomb does more than a STAB Dragon Claw. A Fire move, on the other hand, gives you an answer to Scizor and Ferrothorn.
BUT, Fire Blast > Flamethrower for Chomp. His SpAtk isn't that great, you need the extra power. Fire Blast also isn't as inaccurate as the comparable moves of other types like Blizzard/Thunder, to where the power makes up for the accuracy dip in most situations.
Things get more interesting if you compare, say, Stone Edge to the fire move. Rock Tomb is just too weak to be worth it.