There are few aspects to look at
First would be guild. Do You enjoy to raid with them? Like on some VOIP application or simple in chat. If You do then probably it would be worth waiting to prove yourself to them as a tank and take that post in longer terms. If You don't enjoy their company and stick with them only to be able to raid then probably You should go for chance in other guild.
Second would be You. Which part of raiding do You like most. Just tanking and running around with shield or playing with other people. If members around You doesn't meter so much for You then I think answer is clear.
Main point is - game is created to enjoy You and You are paying for that then why would You do something You don't like?
If at first you don't succeed, you are running about average