It's obviously not a slam dunk to leave. I'm guessing you like the people.
I think it comes down to one question - are they working to get get better? Is their gear well itemized? Are they working to master the best spec (e.g. are they playing arcane because it's easy, or trying to learn fire?) Do you have a core of 10 or 25 that you think will grow?
If not, then what would you gain by staying? That's the question - what is it you want. Do you prefer the people you're with right now and will suffer the slow progress, or are the wipes getting to frustrating for you to continue?
No one can answer this question for you. We can only try to help you understand it.
If there are a number of replies, you'll probably see a lot of "Dump the guild and join the progression guild" simply by the nature of the members of tankspot - they are min-max-progression oriented posters. So take posts like that with a grain of salt. The point here is you need to do what is best for you. At most we can only help you understand the issues more clearly. You need to make the decision.
Hope this helps.