Joe,
I have a friend with a Gulfstar 37 and he is quite fond of his boat, but it is not the performer that the Tartan is. It has good accomodations and perhaps a better layout although I personally do not like the cabinetry in the Gulfstar as much as the Tartan.
What year is the G.S.? Pre-1980 boats were not as well constructed as post 1980....see this excellent write up:
http://www.jordanyachts.com/archives/2645With the Gulfstar 37, I notice that there is a low spot around and outboard of the cockpit where water pools. I am not sure if this is indicative of the model or just the one I have observed.
As far as value is concerned, the GUulfstars tend to sell at a lower price range over all of their models compared to the Tartans, but maybe this is due to their poor early models influencing the sell prices of all of their boats including the better quality late boats.
Good luck with your decision!
Tom