All I can talk directly about is my RP500 and it sounded great directly into a PA. I'll admit - I've since sold it as I've gone almost completely ITB modeling except for the occasional miking of my VOX, but I really enjoyed it for the year or so I had it. If it's between the 500 and the 1000, I'd probably take the extra cost and get the 1000 for the expanded routing capabilities, but as far as sound goes, the 500 and 1000 probably sound alike. As for the comparison with the POD line, all I can speak to in this regard is that I had an old POD XT and I liked my RP500 quite a bit more than the POD , but I'm also weighing in the fact that I used it also in effect only mode for use with a real amp through the effects loop and it also excelled in that fashion.
Your mileage may vary.
I suggest you attempt to try one out prior to selling you X3 - but I think you will like it. It should be noted that the X3 has the benefit of the dual amp configuration that the RP line does not if that matters to you. With the RP units you are locked into a single amp, single cab configuration.