I'd say only roll a sham if you want to heal.
Mages need less support to be effective dps, and are therefore more favorable as a pvp dps class. The shields, poly, snares and roots allow you to do well solo. Basically, mages just get a hell of a lot more survivability and cc.
Shammies only get shocks, and nearly all their casts are easily interrupted. An elemental shaman has massive burst and dps, but you really need the support of a team to be able to get the time and distance you need to cast.
Melee shamans get kited way too easily to be worthwhile in pvp, also, the bulk of their burst relies on a (I believe it was around 16%) windfury proc.
Resto shamans are good healers though, and have all the features people like about shamans in general, but can last longer without running oom.
I believe you'd level faster as a shaman.