If you're concerned about hash collisions your best bet is to use a better hash algorithm, if possible (e.g. SHA-2 or SHA-3).
However, to answer your question: given two values, if you XOR them, you may see collisions even before you do the hashing. For example, 1110 XOR 1111 is 0001, and 0011 XOR 0011 is also 0001. On the other hand, concatenating the values can't introduce pre-hashing collisions. So I'd concatenate.