I know it well, we're neighbors in fact, my apartment is in Walcott St. I'm moving back to live there in September, gonna wait out the Aussie winter up here.
I suggest you scout around on a motorcycle for a few hours during that first stay on Samui and find somewhere you like and book it for the return stay after Koh Phangan. You asked about MaeNam before. One real nice resort I stayed at a couple of times is Paradise. They have a big pool so we weren't interested in the sub-standard beach. I think they charge around 2000Bt, very nice place.