I just found out that in Grizzleheim and Wintertusk you get few spells for your class. I am at the end of Moo Shu and I am wondering is it worth to start doing Grizzleheim before going to Dragonspire.
Level 43 wizard
It certainly is worth it. Grizzleheim quests help you level up. More levels means more health, more mana, and more available spells. You're most DEFINITELY going to need all of these things for Dragonspyre.
You can do it before DS, after DS, or at the same time -- jumping back and forth. The Grizzleheim part will be easy for you at your level. Ravenscar ( a part of GH) is about your level now. Wintertusk is a bit ahead of you. But at least finish GH.
It is absolutely worth it. I'm a level 87 wizard, and I remember wondering why I didn't get certain spells (as a menu chat, I couldn't ask other players, and I didn't exactly realize that I could ask questions on these boards). Lately I've been working on my level 45 wizard, and as an experiment I decided that I wasn't going to do any side quests other than Zeke's quests and the spell quests that our teachers give us. I've finished Dragonspyre and I'm stuck at level 45 -- I can't get to Celestia for another three levels, thus I've decided that I'm going to be doing my Grizzleheim and Wintertusk quests to help level up. Now I'll be a higher level, more powerful, new spells, and it's a nice break from the challenge of constantly being less powerful than your opponents. Plus, Grizzleheim and Wintertusk are absolutely amazing, I love these worlds (they are my favorites, after all), and the storyline is fantastic. I suggest you give it a try.
When you first start Grizzleheim, it is mostly designed for players level 20-30. This is why the XP in these areas like Savarstaad Pass and Mirkholm Keep have XP from the 100-500 range. While questing on my myth wizard there, I was happy to see that it took me just a couple hours to get through all of Grizzleheim and to Wintertusk. But just be aware that Wintertusk is a huge step of difficulty compared to Grizzleheim. With the spells and the XP, it is worth it.
I would recommend doing dragonsypre until you're lvl 45, get some good 45 gear with damage health power pip %. Don't forgetting mana! Resist would be good also but anyway you should do grizzleheim! The xp may be low, but the gold reward is higher! Also the robes hats etc sell for more than your average Mooshu robes and stuff. You will benefit greatly especially in wintertusk. The xp is HUGE!!! Everyone should do grizzleheim!