I'd say GSPlayer sounds the best especially when combined with X50mix.
GSPlayer has a better equalizer than TCPMP and more sound adjustments like pseudo sourround and reverb if you like. TCPMP is more video-oriented than audio-oriented, and while it supports a few more formats than GSPlayer, I'd still say GSPlayer sounds better as it gives you more tweaks. It seems rock solid too.
Both are equally excellent players, mind you. I use GSPlayer for most of my music and TCPMP for videos and formats GSPlayer can't play.
If you want to try GSPlayer, check out JMisurda's VGA version if you have an X51v:
http://www.cs.pitt.edu/~jmisurda/ppc...er2_xscale.zip
Here's his site for further info and more X50v/X51v optimized programs that he;s worked with:
http://www.cs.pitt.edu/~jmisurda/ppc/#GSPlayer
Give it a try! :D