In short, you do not want only one of whatever device you have (and for that matter, not just two), so unless you can sell both to someone who seriously needs them, keep them but they are seriously high-performance devices suited to more complex tasks. For simple experimentation with sensors and such, the Arduino (Pro Mini/ Micro/ UNO) is more suitable.
The Propellers are useful for things such as Web servers - which you could do with a UNO - "sort of". And the important thing is of course, that you have them on a board, as the chip on its own is not very easy to use.