What kind of a place are you looking for? As Nick wrote, PJ has lots of action, including many bars and restaurants. Mt. Sinai has none. If you want quiet, go to Mt. Sinai. If you want night life, go to PJ.
PS. I have seen boats anchored in Mt. Sinai. You need to go all the way in, past the town boat ramp. There is plenty of water, but I don't know how much room. If you want a mooring, Ralph's Fishing Station has some available.
thanks barry - mt. sinai worked out perfectly as we did want some quiet. we are planning on being back this weekend.