cerastium hands down

That's funny, David, since I was going to say the catmint. I have to beat back my catmint which grows vigorously, roots where stems touch down, and seeds modestly, but smothers everything it grows near. In my garden the Snow in Summer/Cerastium doesn't get tall enough to emerge from under the Catmint/Nepeta which makes a 3'x3'x2' tall mound before it starts stem rooting and seeding. Cerastium always sort of faded out in the summer here, though it would return in the spring. I guess it's differences in climate and soil.

I vote for snow in summer.

I was going to say catmint too. I've had both and the Snow in Summer lost a lot of it's foliage after winter here and looked so scraggly that I took it out.

Well so far it is the Snow in Summer ! I was going to let them mingle together but remembered the catmint dies out in winter while the snow in summer does not. So that would not look very good. But the snow in summer is winning out over the catmint. Amazing!