I have the air version of the 3x4 surfprep and it is really handy for certain things. I have a compressor that can handle an air sander, but did the math on the electricity use, and it would pay for me to buy an electric vs air. As far as the Rotex goes, I am not sure it would be worth it for me? It sounds like you have a lot more experience then I do on that.
I build a lot of cabinet doors, and they get widebelt sanded to 180, then I sand them at 150 with the RO sander to remove the cross grain scratches. This takes me, on average, about 5 minutes a door, but about half of that time is easing the edges and hand sanding edge profiles.
Back to the sanders-I would just buy the Surfprep-but it is over $200 more then the Airvantage...