Hi EN0,
I myself think that PIC32's are the ducks nuts
I really can't fault them at all. They have a bucket-load of RAM, heaps of program memory, lots of speed and many useful peripherals.
Like I said in that other thread, PIC32's are excellent for running LCD's due to the DMA and PMP. If you have a picture (for example) in memory, you can draw that picture to the LCD itself using (pretty much) 0% CPU power. You just set the DMA and tell it where to read from (and write to) and then hardware does the rest for you. The DMA will also work for reading from SD card with very little software intervention.
Also, there are excellent libraries for the PIC32 ranging all the way from standard FFT up to voice recognition. The C32 compiler is based on GCC and is free, too.
Downsides? I haven't actually come across any yet but I'm sure there are some.
I understand I probably sound like a PIC32 salesman
but needless to say, I am impressed by them.
I suggest you have a look over at the microchip C32 and PIC32 forums. There is a lot of information there.
I guess what it comes down to is personal preference. If you are familiar with and like PICs and MPLAB, then I see no reason to change.
Edit: Also, for your purpose there is an **broken link removed** describing how to port a software MP3 decoder to the PIC32 which may come in handy.