Avia 500/600 are the graphics driver chips. The older dboxs have avia 500 chips and the newer ones have avia 600 chips. Most people say that you can't tell the difference between the two. You may argue that the avia 600 is better because it's newer but it doesn't really matter what type you get.
It's a bit like saying that a Nokia box is better than a Sagem box or vice versa.
It doesn't make sense to blame the avia 500 chip for the clock sticking! Have you used a splitter to run the two boxes from the one cable feed? Maybe that the avia 500 box has a weaker tuner and needs a booster.