Personally, I think a Christian agent could more sincerely get behind what it is I have to say; he/she may also have more contacts in the Christian publishing field, too. But I think alot of it depends on what it is you write. If you write for the crossover market, whether or not your agent is a Christian may not be as important.
One thing to keep in mind is that not all writers need an agent in the first place. (I don't, for the type of writing I've done so far.)