I want to join an organization to defend our 2A. Sadly due to current budget strain, I can only join one organization. GOA seems like the one with the most potential. Or perhaps I go with state specific group such as ISRA. Is it worth join over GOA? Can any member here share his experience?

I'm a life member of the ISRA and have been since the early 1980's.  I was also member of the ISRA range in Bonfield IL. for many years until I moved to WI. in 1993. For a state organization and a NRA affiliate, I found ISRA members to be highly motivated and ISRA leadership to be very proactive when it comes to securing 2A freedoms in Illinois. I felt then and still do today, that contributions spent on home state issues are always the best way save 2A freedoms in IL.
Gary

Right now, the best bang for the buck is Second Amendment Foundation.  Their lawsuits are the only needle mover we will have.  They totally foot the bill for most of our lawsuits including the ones for ISRA.  
ISRA could be good but, need reorganized.
NRA needs us but, they need to clean house too!

I belong to goa and saf, life member of isra and a range member.  Yes I think it is worth the membership

Yes, they seem to work well with the Second Amendment Foundation in IL.
They actually invest in lawyers trying to get rid of ridiculous IL laws rather than just griping about them.
Sign up for their informational emails, as they keep you up to date as to what is happening in the IL state legislature.

I am member of ISRA, GOA, SAF, and NRA.

I think the ISRA is worthwhile although they are not perfect and I may not always agree with policies or approach or tone in public releases, they are "local" to the state, partner up with other organizations like GOA and SAF for lawsuits, etc. provide updates although I would look to IC for legislation updates and alerts. We cant rely on the NRA like we used to.

Illinois folks need to get more involved with legislation and their reps.  With 2M + FOID cards and the ISRA having only something like 25K members iirc we need to be more of a force.

IIRC, the ISRA membership also meets a requirement criteria if you are looking for things like purchasing from the CMP.

Yes, it used to at least.

ISRA is good if not great.  Plus you can then join the range if you are reasonably close to Bonfield, IL.