MJB,
to the forum. You're sure to get lots of different opinions about your tow vehicle and trailer choices.
Essentially, between those two RV's, I think it boils down to what you like and can afford. Basically both the RV's you mention have the same GVW, tongue weights are within 100 lbs (dry) and although the floorplans will lead to a different loading configuration, if you are careful in how you load, you can keep tongue weights nearly the same with either RV.
If you're happy with your current rig's performance, you should be just about the same weight and tow configuration with the new one. The biggest difference, I believe, will be the overall length. You may (then maybe not) have a bit more sway with the longer RV, and might need to address that problem with hitch modifications ???
Overall, I think you'll find either RV will tow about the same behind your truck.
Good luck with your decision, look around the forum and I think you'll find tons of good information about RVing in general and more specifically about Keystone, Cougars and even the model you have as well as the one you want...