Level both tank classes?

[Lemon8or]

Should I have a decent idea how the classes will play by level 30?

Probably not. A lot of the better core abilities (Inner Beast, Infuriate, Storm's Path & Storm's Eye, Sentinel, Hallowed Ground, etc) are at higher level. I would level both to 50. Since tank queues are so short, you can level them both in dungeons and be pretty quick about it.