In early raiding, elemental shammeis can easily top charts. Pushing 1100-1200dps in nothing more than T4 purples, maybe not even with badge gear, is possible. Enhance shammies can't say that.
Enhance shammies scale better though. Take Brut for example - our enhance shammy breaks 2000dps, and that's dropping 100dps or so by not using Flame Shock (we're debuff capped on Brut :x). I might be wrong, but I don't think an elemental shammy can quite put out that much with equal gear (we don't raid with elemental shammies much atm, those we've recruited are either still gearing up or... well, leave things to be desired).
On the other hand, a resto shammy can provide 80% of what an elemental shammy bring to a group. However, compare a resto shammy's group buffs to melee with an enhancement shammy, and it's a lot bigger. Extra AP, extra Windfury, 10% more AP, and if the enhance shammy is good, an extra 88 agility too that the resto shammy isn't brining. All an elemental shammy has over rest is 3%crit 3% hit, which isn't anywhere near as important to have.
I'd like to have both in any raid, but if I had to choose between the two, enhance is better, especially as you get into T6+.