I read Ringworld just recently and was amazed at how nothing happened! I mean sure, there was a cool world (Ringworld), an interesting interplanetary/interspecies civilization, and a cute concept (breeding for luck).... but still nothing interesting happened. It was quite disappointing.
Dune on the other hand, was rich in all the ways Ringworld wasn't. Epic story, epic quest, complex civilization/politics, and adventure!
This was an easy choice for me.