Tough question. I really like both patterns but they're very different which doesn't help in a head-to-head comparison.
One thing I'll point out is that although the dimensions might not sound very different on paper (or a screen...), the 72 is a more substantial knife all the way around. Depending on your intended use, that may factor in for you.
Here's another thing to consider, as
H
Hoffmajo89
pointed out, slipjoints are GEC's bread and butter and I'd certainly suggest you own at least one if you're even remotely interested in that sort of thing, but -- I know for certain that GEC is gonna produce more slipjoints, as in they've probably already got the next slipjoint pattern they'll be making in production now. When they'll do another run of lockbacks is anybody's guess -- maybe next month (unlikely), maybe next year, maybe in three years, maybe five, the point being that no one really knows. If you think you might want a lockback from them at some point, now might be the best time to score one for the foreseeable future.
Because threads are useless without pictures, here's an image of both side by side. The 72 is Che Chen Rosewood, and the 56 is Orange Scramble Acrylic. Apologies, but I wasn't able to score an Avocado Jigged Bone 56 but I wish I had.