You can't go wrong either way really. If played well, both will work just fine.
Just because Paladins have cleanse, Shamans have Purge, the exact opposite really. Many crucial abilities can be purged, such as Earth Shield for example.
Shamans with their Earth Shield, have 100% uninteruptable healing and their 1.5 second heal, although more mana intensive is much more powerful then flash of light. Shamans also have Natures Swiftness, which can be used for an instant powerful heal, or even used offensively (NS + chain lightning).
Paladins are able to cleanse sheep though, which is an advantage over Shamans. Shamans totem buffs are stationary, you can't leave more then 30 yards away and have to micro manage them if they expire or are destroyed.
Grounding totem is very nice too against those pesky pom mages and of course all casters. Paladins bubble is very nice, although watch out for those Priests which will dispel it!
I don't like Flash of Light very much, although it's very cheap, it doesn't heal for much unless it crits, which could be a problem against high dpsers. So keeping your buff up for holy light to be a fast cast is another important thing too. Three seconds to cast is just asking for an interupt.
Just because Paladins are wearing plate though, doesnt' make them hard to take down at all, With Earth Shield(again if it doesn't get dispelled) it makes the Shaman very hard to take down.
My two cents :)
Maybe also check out the top teams and see who their using as their healers more?