I'm not aware of any blaring changes besides the 30DS having the XPackage available. Which might be worth it depending on the price discount on between model years that your seeing. To actually convert it over to 50 amp setup with a second AC, I'd think it'll cost $6-7k going the Onan 5.5KW route. We have the 30 amp setup and we'll probably add a second 13.5 Atwood AC unit to the rear and run the extension cord down to be plugged into the 20 amp outlet at a pedestal. If I need to run it on the road/boonies, then I might mount a 2KW gen somewhere if it won't run on the 4KW genset.
My recommendation if you go with the 30 Amp rig, would be to get the dealer to install at least a 2nd AC in the bedroom with an extension cord run down to the power bay prior to delivery. Should be an extra $1000, but it would be worth it.
2015 FR3 30DS