I had a MK-IV trunk mounted AC unit, but it wasn't charged when I got the car and have had no need for one since so I can't comment on how effective they are, but they do have two blower fans and have ducts that push the air onto the rear seat passengers blowing forward. Windows up no problem but if you're the type that enjoys air on with the windows down the cool air probably won't make it up to you very well. If you're like me and rarely carry people back there it may be a more "comfortable" and diluted cool air feeling, but would also lower the blower noise you would get having one right up front with you all the time as with the underdash models.
Trunk mount also means less trunk space, but how often have you really needed more in these cars?
I also personally prefer the looks without the underdash models, but that's highly personal. The advantage I see with underdash is no routing of long hoses to the rear. Also, this stems from my ignorance about AC systems but I would think you would need to fill those hoses with the coolant as well meaning you would spend a lot more to fill and anytime you got a leak.
Whatever the case consider replacing the compressor, the modern ones weigh about 1/3 or less and are much higher efficiency. Take your old one apart and make some desk art out of the tiny pistons.
