I've done plenty of both and they each have their advantages, as others have noted above.

I think the ones I enjoy most are not the super-organized Event level centuries (Palm Springs, Solvang, Wildflower, Lighthouse, etc.) but the ones put on by a few cycling friends who just decide to do one, without any outside support. That way we can take all day, re-group whenever we want, stop to eat at some "sit-down" cafe or just buy food along the way.

In these days of email and facebook, it's fairly easy to get a group together for a century.

I've never ridden a double on my own, but as John E. points out above, sometimes the separation between riders is so great that it seems like doing in on your own.

Overall though, I like the small informal centuries the best.

Rick / OCRR