1x Priest , 4x Mages = easy to level with aoe grinding /aoe questing. Downfall is you have no tank limiting your group to BGs, and it narrows the normal lvl 60+ instances you can do. On the plus side you do have 5 forms of CC which limits the demand for a tank. I don't know if you can do heroics haven't seen any done with this combo, and from what I've read it's not that viable in arenas. I'd say it depends on your personal skill as to how you do in arenas , but I do know from running into this combo in BGs that it is pretty nasty vrs my 5 sham team.

1x Priest, 4x Warlocks = easy to lvl with the instant cast dots and running around pulling in some insane amounts of EXP/Hr. The warlocks bring pets to the table giving you either 4 seduces for CC, or 4 tanks, or 4 semi tanks with mad dps. This combo does some serious damage in BGs, but they lack the BURST for a successful arena team from what I've seen and heard from those that I've asked.

1x Paladin, 4x Sham = easy mode instances, and I'd assume that this isn't a bad combo for pvp considering the 4x sham. You can do normals, heroics, BGs, and possibly arenas though this would be tricky 5 boxing.

I tend to lean towards the 5x sham because of the viability in all areas of pve and pvp. They have the ability to kite and partial tank almost any instance pre-66, and then when you get earth elementals and fire elementals the heroics and the higher end BC intsances become easy mode. However, I've noticed that when you play 4x sham and then get a 2nd person to be your dedicated healer your win ratio in 5v5 tends to be a lot better. Knowing this I'd probably roll the 4x sham and 1x paladin so you have dedicated tank if needed, and you could always arena with your 4 shamans + 1. Then 2v2 or 5v5 with your paladin solo for phat points.

The major downfall that I see to Shaman teams is the lack of CC, but with the BURST dps that they bring to the table along with totems it is a decent trade off. With an AOETankadin CC isn't really an issue. With 5 shamans again with stoneclaw and earth bind the need for CC becomes a moot point.