Personally if you can afford it at all I would go for the $600 bike. One used at the YMCA is likely to have had some pretty rough wear and LOTS of use. And the three year full warranty is worth a lot. Just my opinion. Hope you will let us know what you decide.
I love spinning... used to do it when I lived in Fort Lauderdale! I need to check around to see if I can find a class around here!