AFAIR Brilliance can work with 24bit pics better than the other 2. I think DPaint only started to support 24bit by version V. PPaint can only work in 8bit but has very good color reduction routines that make up for building very good palettes. For sprite making you'll only need 8bit though. Hey why not try out all 3 yourself ? They all can save as IFF so your work is portable between them.
Then there's animation support, I think DPaint is better but haven't used any of the programs extensively for that so you might be better with someone else's opinion on that one...
