I go to Sport Club -- and I love it. You're correct, Killarney. Paying with your credit card means you'll pay $250 ARS for just the local club access, whereas you'll pay $300 for access to all clubs. If you pay in cash, they double it. I have no idea why, but I assume the CC price is an incentive so that they make sure they can get their money every month.
My partner and I both pay $300/month (each), and, as far as I'm concerned, it's totally worth it. There are tons of classes to take; the equipment is top notch; and the location I go to in Belgrano is always pretty peaceful. It's also two blocks from my house!