I like them both but it's hard to say without knowing your other villagers. I have Lopez right now and prefer him for my town because most of my villagers have some sort of brown or yellow so aestheticly he fits in much more than Zell would.
However in general I do prefer Zell just a little bit more. It's very close though.
I prefer Zell over Lopez, not a big fan of the deer species but out of the two Zell is the one I prefer. Imo, I think he has a nicer colour scheme than Lopez and a nicer house.