The Sony VTX has got great reviews, second the Pioneer. Picture quality and sound does vary amongst models but not too a great extent - some models give a softer image but the Sony and Pioneer are both excellent. One thing to watch out for is how fast they can change channels - the Philips model is allegedly quite slow in comparison.
By the time top up compatible boxes are around, they will be tits up. For plain and simple picture quality, if that is the only criterion, it's Netgem. I have seen almost every box on the market, quite a few one after another. For other things, another might suit better. I actually own a Pace, though I have netgems pretty often, they are a pain to set up on site, so I set them up at home. They go on the phone line for 10 mins, and need internet features set up before they can be used. One point to bear in mind is their sensitivity to poor signals. Sometimes moving them on the rack can help, they like to be on the top. (knew a girl like that once....sigh)