I never tried Stroller Strides, but my daughter has been going to Gymboree since she was 6 months old and she loves it. She just turned two and we are still there. In addition to the Gym type clasessed they also have Music and Art. If you live near a GYmboree I woul reccomend the Choice membership. You can go to as many age appropriate classes as you want as well as the free play times. We live close enough that we are there almost evey day, and it is more than worth the cost. This term choice membership for the 15 week membership was $650. That works out to about $43 per week, then the more classes you go to the better the bargain you are getting - I try to get to 9 or 10 classes a week. Plus if you are a member they have a loyalty program which give you dicounts on the cost of the enrollment and they often have early bird specials (too late for this term)which give great discounts (it cost me about $3.50 per class with all of my discounts).
I also saw something in Parents magazine for $50 off enrolment and $25 in free merchandise. I think if you mention then they may be able to help you.
Obviously I love the place - I go to the East Northport location - all the people there are great - I think it was the best choice I made as far as entertaining my daughter!