Remember their typing. The only significant difference between these guys is their physical and special stats.
Solrock-The physically oriented of the two. he has some good physical moves like Earthquake, explosion, and return.
Lunatone-the special rock. some good special moves include psychic, ice beam, blizzard, shadow ball, etc.
Both of them have a ton of weaknesses and have as much potential as the other, given the right moveset. To be honoest I prefer Lunatone since he is a special fighter, but that's just me.