First off, make sure you have a good, snug fit. You need the BC to hold the tank steady on your back, or you will be uncomfortable in the water. Second, if one of them has two cam bands (and I'm completely unfamiliar with both), that would be a good feature. You want the tank stable, and two cambands gives you greater security AND greater stability. Third, see if either has trim pockets up on your back. Having trim pockets gives you an easy way to move some weight around to balance your gear.
I would suggest putting the BC on WITH a tank, and inflating the bladder, and making sure you are comfortable in it with the bladder well-inflated, as it would be at the surface. Depending on how the bladder is constructed, it may or may not constrict around you when inflated.
I would also highly suggest that, if you buy one of these, you either take the inflator apart and service it (if you are the mechanical sort) or you take it in to your local shop and have it done. Inflators do get crud and corrosion in them, and o-rings age, and a leaking inflator is at best annoying and at worst a recipe for an accident. You also want to make sure the BC will hold air, which is best done by inflating it fully and letting it sit for several hours. You may not be able to do that with this sale, but at least inflate it and let it sit for as long as you can while you are talking to the fellow who is selling it. You CAN patch holes, but you really don't want to buy a BC with a hole in it, if you can avoid it.
