I play both pretty consistently and when I'm on my Druid I miss having kick and totems, and when I'm on my shaman I miss having the mobility and instants. Shaman is gonna be better 1v1 (burstier damage and the kick) but in bgs... if you made me choose I'd pick my Druid.
If you're looking to build and only play one, I'd recommend druid. They are "forever meta" in low level pvp. And if you ever do decide to dig into the achievement hunting part of F2P, then druid is hands down the best class to do it on.