I don't have both myself, but from what I gather... the interface is pretty much the same. The XM is more integrated with the head unit, which means you get the channel icons on the screen. But the number of display lines and what you can see on the screen (artist, title, etc) are pretty much the same. They both let you navigate directly to a channel by number or scroll through each number or scroll though genres, etc.
I have the Sirius PNR1 installed in mine and I don't really find any huge negative with it, or even enough to warrant me going to the PNR2 either. I suppose it would be cool to have the PNR2 functions, but I would honestly never use them. It would be even cooler to have the icons show up for each channel on the screen, but not enough to give up the programming I have come to like on Sirius.
Best thing to do is look at both and look at the programming they offer. Personally I like Sirius (Hair Nation, Buzzsaw, Octane). My wife's Acura has XM and not sirius and after driving her car enough I don't think XM has anything close. Mostly they are the same stuff, but each has different programming that the other doesn't. I wouldn't pick one over the other based on the interface, I would pick it based on what you want to listen to. I have what is reguarded as "the worse" interface or integration with the Z1, the PNR1 and it's really not that bad. There are a couple things people said and showed me which are included with the PNR2 or the XM one and it's nothing I think I would miss. I would miss listening to Sirius more than I miss the extra "pretty" things XM can do.