Concatenation of the hashes seems like an unjustified risk, that in certain circumstances will allow weaknesses from either algorithm to flow throw to the final hash. If you really want to combine the hashes, XOR seems like a safer bet to me (since the algorithms are unrelated there should be no potential cancellation of entropy).
Seems like XOR is better for approximating a random oracle, and appending is (negligibly) better for ensuring collision resistance. People often are not clear about which of these two very different properties they actually want out of a hash.
