If the tablet still works fine, I'm not sure why you want to get rid of it... But if it's really uncomfortable to use and stuff, I'd go with the new tablet. But if you're still fine with using your old tablet until you have more money for one, go with the MP3 player.
If I had to pick, I'd choose the tablet because I already have an MP3 player that I like and is new and blah. I know, I'm not much help at all.