Obviously if you want your playback device to do more than just play backing tracks (for example, provide sheet music or lyrics) then your best bet is a PC or tablet.

The advantage of a PC is that they are often less expensive than tablets, and there's an abundance of music software and playback apps available for notebooks. (like the one Bob named. )

Tablets are more compact, and you can buy brackets that attach to a mic stand to put all that functionality at your fingertips. Its a more compact system because you don't need a keyboard or mouse. There are now interfaces for tablets so you can get high quality sound and not have to rely on headphone jack output. Apps may be limited compared to PC, but I'm sure there are still enough to provide quite a bit of choice.