The Cosmos S is a nice case, not my favourite but it is good.
The Antec 1200 is great and I'm not saying that just because I have one. It doesn't look the best in pictures but up close with all your components in it looks cool. For me the dust isn't a problem, I'm sure you are aware that the front 3 intake fans come with dust filters. They don't restrict airflow a lot and they are easy to clean. I really don't think that will be a problem. Also the optional side fan comes with a dust filter too.
The airflow through the case is really good, my 260's are idle at 40,44 degrees with all fans on low and my cpu is 24,23,19,19.
Overall a good case and well worth it.