I've used both a 50 & 51 to output composite. The 50 does simultaneous output through its component, composite & S-video connections (the latter 2 downrezed, of course). On the 51, you have to select either component or composite output (no S-video at all) using a back-panel switch.
Like Chazz, I like to be able to preview material on a small monitor (while recording or to cue up for playback), so I find the 51 doesn't suit my needs (& will probably sell or trade it).
Hope this helps.