I've done both. I MUCH prefer OBR and am signed up again this year.

OBR is smaller, and much more personal. Camping is in school fields, city parks, etc. Food is generally very good. Routes were excellent. I disagree with TsPoet's assessment that CO is harder or hillier. I think it varies by the year. Last year, OBR had 35,000 feet of climbing (which includes the optional rest day). It was a very challenging ride. It's an older demographic on OBR, so they freely sag the folks who can't do the distance or elevation. I rode it all, and it was an amazing ride. CHallenging, but amazing.

CO has its perks (nightly entertainment, etc.) but to me it's just gotten waaaaaay too big. Such a big carbon footprint with the stage and the vendors, etc. And at that size, they're mostly limited to farmers fields way outside of town. I prefer the small size of OBR - it means that after a shower, I can stroll into town and get a feel for the place, instead of having to ride 5 miles into town fromthe farmers field.

YMMV.