As far as I know soundwise there is no difference between the HD500 and HD500x it has just a better CPU. Some people have the problem to run out of DSP Power with the HD500 when using lots of effects. I personally quite never run into that problem, 'cause I don't like overloaded tones anyway. I like to keep it simple.
I had the HD500 since it came out and I know that it can sound very well. I read a lot of complains on the net, but I think people expect to plugIn the guitar and it sounds great. This isn't the case with the POD. You have to tweak a lot, really a lot - but It then it can sound really good.
Never looked for a another device since it fits my needs. Latley I feel attrackted to the eleven rack. Maybe some day I'll get the change to test it out ;-)