Your anything but underleved to me. ^.^ I was level 23 when I started, and I finished at level 29. :D
Anyways, Grizzleheim gives you less exp then Wysteria, but Wintertusk gives you about the same exp as Celestia... So, it's really your choice, do you want exp NOW, or more exp LATER? So, basically what i'm saying is: Wysteria has an average amount of exp, and you can get it right when your questing, but Grizzleheim basically gives you like 90 exp per quest, unless it's harder. (instances, dungeons) So, after you finished Grizzleheim, you can do Wintertusk, which gives you alot of exp.