The major difference between the XLS & XL is the seat. The XL has an air bladder seat...it's quite comfy too! The XLS has a foam seat that is supposed to be as comfy or more comfy than the air. The reason for the change(or so I've heard) is because the air bladder seats would begin to lose air or puncture easily.

I have a Bushmaster XL and a VIper XL. Both have the air seat, and neither one leaks...I've had them both for 3 years.