Those are two different things.
ECC is an Error Checking Code-its a feature that basically checks for errors. Its slows down the RAM module, so its usually only used in servers, where one tiny error could bring down an entire network. It also makes the module more expensive. Get non-ECC.
CAS latency has something to do with the speed of the module, combined with bandwidth I think, not so sure on it. I think lower is better though.
Good going with crucial.