If I want to connect two networks wirelessly through 1 or 2 walls, what's best? N? AC?

I've got two networks separated by a wall... now the easiest thing to do would be to drill a hole through the wall but I can't do that. I would need to bridge the two networks together. I already did this using a "Access point + WDS" and "WDS" (WALL HERE) and "Access point + WDS" connection with 3 routers.

Currently it's using wireless N on the 2.4GHZ network. Would 5GHZ be better for overall speed? Would AC be better?

I heard 5ghz does not penetrate through walls well but i figure the...

If I want to connect two networks wirelessly through 1 or 2 walls, what's best? N? AC?