this is what i got out of someone on another forum...
Long story short: Rock is a little bit more "appealing" but unreliable and ends up forcing some people to restore their phone afterwords. Cydia is somewhat slower, but does the job the right way and how it's supposed to be done. Rock depends on Cydia, so without Cydia, Rock would not exist.
You won't find any actual technical explanation comparing the two, because it's all a matter of preference and opinion. Just keep in mind, the cons for Rock far outweigh the cons for Cydia.