Both methods only check whether the first entry of the first list occurs in the second list. I highly doubt that your hash method is faster in such a case.

However, it was my understanding the OP wanted to know whether the first list contains entries that aren't in the second list. This is a FAQ, and a hash solution ought to be used, as that gives you linear behaviour (assuming all your hash inserts go in constant time on average). A grep method will be quadratic, and for any list that doesn't have a trivial size, it should not be taking into consideration. After all, both methods use a linear amount of memory.

Abigail