imo: (I have 5 shaman and 1 pally by the way)

If you like to PVP mostly - 5 shamans. there's really nothing like 5x CL. 4 is nice - but 5 will knock 2-3 people out a lot of times. You can either make them all elemental or make 1 resto. Play the elemental in a 5's with a dedicated healer and play the resto in 2's 3's or 5's solos for fun. When it's BG time they can all group together.
You can use them all to do heroics, probably a bit better and easier than with a pally tank for the most part.

If you like PVE mostly - and want to do some non-multiboxing raids, it'll be really nice to have a pally tank. they are usually in high demad and can get good gear fairly quickly by raiding. Also, you can do ok in instances. I actually did kara this week using my pally tank to tank, and my 5 shaman to DPS with. It wasn't that bad at all.
It's also nice to have crusader aura and it's probably a bit easier to level up everyone with a pally. That's how I leveled 4 of my 5 shaman.