So far, it seems to me that gst123 is much better that Yauap since gst123 can handle video files too and it can handle Internet streams, unlike Yauap which doesn't even handle raw files (i.e. http://www.salixos.org/audio.ogg).
On the other hand, Yauap seem to consume less resources that gst123 and it is displaying the metadata (tags, genre, year etc.) which can be beneficial sometimes.
Overhaul, I think that both of these players are inefficient. Mocp and Cmus take between 2 - 4 MB (maximum 5 MB) whilst gst123 and Yauap take between 7 - 12 MB (it is rare for these two to consume ~5 MB).
I do not have much experience with MPlayer, so please feel free to write down your opinion
