Based from reviews on newer trucks, here is what I have gathered:
Very fuel efficient, compares well to the Freightliner Cascadia. You can find these trucks at a cheaper price than the competition, but probably with fewer options. ProStar has a wide range of options to fit the driver's needs, so you would have to shop around.
The new Maxxforce engines are very quiet, the Cummins engine is also very quiet in this truck. The ProStar has a comfortable, "car-like" feel while driving it, and the styling isn't bad for an Aerodynamic Model truck. Interior is very spacious and the sky-rise version will comfortably accommodate two adults. Easy to clean and maintain inside and out.
Good solid ride, new models have a lot of bugs to work out, but what new truck doesn't? Good gas mileage, not really any difference than the ProStar competition. Roomy and comfortably interior if you get more of an O/O package, and prices are also not too high. Easy to drive, large turning radius for a full size sleeper truck. Condo sleeper is comfortable. Only complaints I've heard on the sleeper is that sometimes they have air leaks while driving, or water leaks in bad rain through the top windows.
Both models are very roomy and comfortable. Drivers that I have talked to that drive one for a company say they are one of the most comfortable trucks out there. Cummins engine is the way to go on these trucks. Styling is great, maintenance is about the same as on every "Aero" truck. Haven't gotten any negative comments about them. Prices are a bit higher than on ProStar and Cascadia. Build quality is a little better.
As far as PACCAR trucks go, you really can't go wrong with any of them. It all depends on styling for you and gas mileage. The Aero models will get better gas mileage, but you can't beat the traditional look of the 388/389. The Kenworth T660 is a really good truck, no complaints about the styling or performance of the T660.
As far as my opinion, I like the Volvo's a lot for an Owner Operator truck. Those and the T660 are my top picks. Good luck on the find.