I've stayed at MYC, Montauk Marine Basin, and Star Island (that trip was on a friend's boat). MYC is much more a resort marina than anything else in Montauk, they have 2 pools, a spa, different bars and restaurants, a little beach area, you can take kayaks out, etc. It is very pricey though. Normally when I'm in Montauk I'm there to fish and for that there are better choices since they don't have have bait and ice (in quantities for fishing at least) at the marina. They also put me in a slip initially that I couldn't come and go from at low tide. I also didn't really feel like I was in Montauk when I was there, I like being on the other side where I see the commercial boats and fishing fleet coming and going. I normally stay at Montauk Marine Basin, my boat is there now actually. It's much more of a fishing marina though and not really a family vacation spot, although it seems like more cruisers have been stopping there the past couple seasons. I'm in an outside slip with a great view of the harbor, and within walking distance of everything along the road to Gosmans. Star Island is nice, but they charge nearly as much as MYC without the 'resort' feel in my opinion. Their floating docks are nicer than the fixed ones at MYC though.
John