As for me, now I have only DRK but it's my first tank ever XD
If you played tank in another MMOs, you can try GNB as first tank, but you anyway should finish The Smith lessons and train with dummies.
All tanks are different with own pluses and minuses, so it's better to analyze your gaming expirience. But remember that GNB job let you go to 60+ duties in roulettes so be prepared.