You'd likely need an extended seat for the Sensation. The regular seat fits more like a 15" western with the Hybrid. They are GREAT saddles. They fit really wide horses well, and even have some cut back for higher withers.
Bob Marshalls are great. I have an Endurance model. The Barrel or Trail models don't fit really wide horses well, but he Endurance model seems to. You'll likely need a 16.5" seat size (they fit a little smaller). If your horse has high withers, you want the peaked pommel option.
With Bob Marshalls, you want to get a Skito treeles pad
with the laminated foam option, to give your horse adequate spinal clearance while reducing the bulk under your leg. You can also use a Skito with a Sensation, but you don't need the lamination (it has its own internal weight distribution system, so you just need to create some spinal clearnace). I have a few different Skito pads and LOVE them!
Another treeless option would be a Barefoot in their Size 2. They come in a few different styles and the front pommels are changable, so you can fit a wide variety of wither/shoulder shapes. Some people even take out the pommel and stuff it tightly with wool to make a softer front. The saddle is still pretty stable this way, and it fits those REALLY wide horses better. The newer Barefoots have pannels underneath, which help with weight distribution. You'd still want a Skito pad, but you wouldn't need the laminated foam. The older Barefoots are like Bob Marshalls and are totally flat underneath. You'd want the laminated foam with those. You can find them used at a reasonable price.
There are some treeless on eBay right now and Tacktrader.com. Check endruance.net's classifieds section. There's a Yahoo group just for treeless enthusiats. Quite a few people buy and sell there. Post a want-ad in the messages and check out the sales list in the group's database. Go to groups.yahoo.com and search for "treeless saddle".