Welcome to the forum. The question you asked has been discussed just recently. Here's the thread where you can read some of the differences:
http://www.keystonerv.org/forums/showthread.php?t=15498
If you look at the floorplans, there are many that are the same between the west and east models, however there are some pretty significant differences in some models. The Cougar fifth wheel 27RKS and the 279RKSWE are two that are significantly different. The WE has only one rocker, the pantry is in a different location and there's a countertop across the entire back of the RV. The eastern model has two rockers, the pantry is in the roadside rear corner and the stove is on the rear wall. The bathroom hallway is in a different location however the bedrooms are nearly the same. The holding tank capacities are also significantly different.
It takes a comparative eye, some time and effort to decipher the subtle differences, but there can be only a few things changed or the trailers may not even look the same.....