I have a Trek 7.1 and I often commute 14.5 one way here in the Appalachian mountains and often have to ride the shoulders of 55mph "four lanes". On my days off I ride 30+ miles. Anyway my tire slashed and I have to get another so I'm wanting a more comfortable saddle while I'm at it: I'm 5'11 and 1/2 and way 239 with a semi-athletic build.
They offer 3 sizes, which one would be best for me?

http://bontrager.com/model/08374

Also they offer more seats on the site, is this the best one for my needs?