Which is best?
This is something only you can decide. Best quality sound for backing tracks is always going to be anything recorded from the source material to CD (equivalent MP3 bitrate of a CD recording would be 1411 kbit/s). Would I use CD backing tracks in a live singing environment? No, as the risk of fingermarks on a disk or the disk skipping due to a bump is too high for a live performance in my opinion. MP3's correctly recorded at 256kbps or higher, together with Minidisc (which again uses a form of compression) should both be near CD quality.
Please don't settle for sellers offering MP3's that have a lot of background noise - the bitrate of an MP3 doesn't guarantee quality. If a track doesn't sound clear, it's likely that it hasn't been recorded from the original source or is a low quality copy downloaded form an internet source. Professional backing tracks should be clear sounding, have a complete ending and use instrumentation that faithfully represents the original production.
The difference between listening to an original CD recording and a high bitrate MP3 shouldn't be the difference between hearing a good backing track and a poor one. The differences noted by audiophiles are usually things like 'the cymbals weren't quite as bright' or 'the vibrancy of the flute wasn't quite there'. These may be important differences when one is analysed against the other, but that doesn't really make the MP3 format unsuitable. Very often the comparisons are made using classical tracks because people are listening for very specific tones of an individual instrument.
What really matters is how you feel about your music. If you're happy with the sound you are hearing through your PA system, there's a good chance the majority of your audience are happy too.