I've only tried the 997. When I was first figuring out sizes, I had problems getting the 997 over my shoulders and shoulder blades since they're decently broad. The description for the 988 says that the compression panels are over the chest and upper back, so I could see the complications for someone who is built more like me.
If you do decide to order one, please post what you think. It's hard to know what you're getting with buying a new type of binder until you've bought it and tried it on in my experience.