If you’re tapping RGB after the encoder, I think it depends on which revision console you have. My understanding is that the earlier-revision consoles with Conexant encoders (v1.0 to v1.3) have good YPbPr but meh RGB, and the later models with Xcalibur encoders (1.6) have better RGB, but meh YPbPr (as well as the occasional glitch that you won’t find with earlier hardware).
And then, in order to get separate sync, you need to pick two lines on the AV port that aren’t being used (or that you won’t use), wire up horizontal and vertical sync to them, and then custom-build a DE-15 cable that taps those lines.
Personally, I really don’t like putting myself in a position where I effectively have a one-off configuration that no longer works with off-the-shelf cables. If I had to get RGB from my v1.0, I’d rather use a custom BIOS that output RGsB, use either the HD AV pack (which I have) or one of those Monster cables with the TOSLINK port built-in, and then either connect that to my OSSC or use an Extron RGB interface to get RGBS.
Right now, I just use YPbPr output with an HD AV Pack, because it’s easier overall to integrate into my setup. I have an Audio Authority 1154A, which automatically switches both component video and digital audio, so it’s straightforward for me to run YPbPr component and TOSLINK from my Xbox and two PS2s into the switcher, and then YPbPr into my OSSC and TOSLINK into my AVR, all without modifying anything.
If I went RGB, I’d need all of my devices going through that component switcher to be RGsB; I could do that for the Xbox, but the PS2s would need RGBS for the vast majority of its content (which is limited to 15kHz video), so RGsB is impractical for me. I also don’t want to spend $120-$250 on an automatic SCART switcher, plus another $100 in cables, so RGBS for everything is out.