Thank you all for the info. I'm not married to the idea of an AGM, and in fact would prefer a flooded or gel battery, but $100/battery seemed like it might be too good to pass up. If I could get new flooded cell batteries for around the same price, I'd go that route! My charger is a Xantrex Freedom HF 1800, which has an AGM mode. The previous owner is getting rid of them because he's upgrading to a bank of 8 L16s.
The area I can dedicate to batteries measures about 20" x 26" x 13" (height), though I can get a little taller in part of it. That's really close to the size of those batteries, Maine Sail... I wonder if I could squeeze two of them in.
The GC15 is only 5Ah less than the GC25? That seems like a tiny capacity loss for such a huge difference in price. I might have to get myself a Sam's Club membership... any idea if Costco carries similar ones? I already have an in there