WMA is owned by Microsoft. From looking around it seems to be agreed that it takes less space but may be slightly lower quality than MP3. Microsoft has a fairly lousy record on file formats so in the long term you might find it unsupported.
The prospect of future improvements to the format is not an advantage. You'll only see a benefit if you rip the music again into the improved version (EDIT - and your player supports the improved version!) - and the improvements may not be backwardly compatible, forcing you to rip the music again at some stage.
Really, if space is short I'd suggest using WMA, otherwise MP3 is probably the better choice.
Edited by PaulK, : No reason given.