So is it better to obtain Invisibility or Invisibility only to Zombies?

This is totally dependent of the type of campaign the GM is running and just what sense(s) invisibility covers. However, it may also be a power that has to garnish his/her approval. Consult your GM.

Now, if Paradigm Concepts ever does a living campaign (such as they've done for Arcanis), invisibility vs. Zombies would be a very, VERY, powerful power (it might even not be allowed).

Not all threats are zombies.. and some zombies have different ways to "see" you