SPRecovery is more stable overall...it hasn't had any updates for quite a while, and I've never had it miss or make a single mistake in many, many backup/restore uses. It's more broadly used/supported in the Droid community than clockwork mod.
Clockwork recovery may end up being a little more sophisticated overall, but it is getting frequent (too frequent, IMHO) updates that indicate a lack of stability IMHO...the last I read one known issue was that in some cases it will fail when generating the MD5 at the end of a backup. That has happened to me once.
That said, I'm using clockwork recovery as I've switched over to Cyanogenmod ROMs, and at this point think I'll be staying w/them for a while.
Unless you are going to start using Cyanogenmod ROMs primarily, I'd stay w/SPRecovery.
Clockwork and SPRecovery nandroid backups are not compatible.