I'm fairly happy with the closed cell battery I have now, from Sam's. Sure, there was a price premium, but it wasn't that much extra, there's no need to check fluid levels, and the chance of a spill are close to zero, outside of an accident or vandalism. Since it's closed cell I could safely move it to the rear hatch too, if I ever decided to do that (although it's probably too big, for where I would want to put it).
It's been some years since I researched into putting a racing battery into the rear hatch and have forgotten most of that info. I can't remember the batteries that were popular, at the time, but they were around half the size of the battery the car was using, for close to the same power output (maybe) and I think they even worked when laying on their side. It might have been gel cell, like you said, because they were certainly 3 or 4 times the cost. I think they were supposed to last up to twice as long too.