Oops! Sorry, I glanced right past the RC5 part and voted for SETI.
Having done SETI for quite a while, taken a break to do RC5, and then gone back to SETI again, I can at least vouch for my having taken that route.
Still, for what it's worth, if I had a large fleet currently doing RC5 and I wanted to switch it another project, I'd probably go the OGR route, mainly because the client is similar or even the same, thus there wouldn't be any learning curve of how to configure for another project.
BTW, with RC5, at least you know absolutely that there is 1 correct key; in SETI, we only know statistically that there are most likely aliens out there with signals for us to find! :Q SETI@home is definitely a better candidate for the "endless project."