Firstly, you can't go wrong with either one. I've heard both players and they're both great. I've owned the Planet 2000 for a few years now and love it. As others have mentioned, it excels at PRAT. For me this factor is very important, so that's probably why I like the Planet 2k so much. It's not the last word in detail or dynamics but it does a lot of things very well. Ultimately, it's just fun to listen to. And a great value, too.
The CD-25 I heard at a dealer, hooked up to a modified Jolida(302-B, maybe?) integrated and some SAP bookshelf speakers. A very, very nice sounding system. Hard to analyze just the CD player since I wasn't familiar with the accompanying gear, but a system won't sound good if the source component isn't up to snuff, IMO.
I suppose this doesn't answer your question at all. However, I'm sure that either player would be a substantial upgrade over the Sony. Both are usually easy to find used here on the 'Gon.