As has been noted these are three very different boats.
The Tartan 370's were available using some of the absolutely most modern materials that are available. While the epoxy hull is about as good as it gets in terms of durability, the carbon fiber mast is a bit of mixed bag; great for sailing ability and stability, but potentially at risk for wild electrolysis problems and lightning problems that would be far in excess of those expected with alumuminum spars. I am not a big fan of the Tartan hull form and really don't like its rig and deck layout for ease of handling.
I really love the X37. I think that the layout is my ideal for single-handing, or a couple cruising, or cruising with two couples. The hull and keel design would be an excellent choice offshore, or sailing in changeable conditions, offering good motion comfort and an easily driven hullform. The sail plan on the X boat would be the easiest of the three to handle and I have almost always been impressed with the quality of their deck hardware and its placement. I really think that X-Yachts does a great job with their engineering, especially since they tend to engineer around conventional materials. That said, I have never felt completely comfortable with the galvanized steel structure that they use for their keel connections. I have talked to quite a few long term owners who swear that it is no problem and it makes a very sturdy boat so I am starting to come around on that point. The X37 is part of their cruising series rather than their racing series and is no more racing oriented than the Tartan although I think it would be an easier boat to race in terms of really great hardware layout. I'd personally would probably do the two cabin/ single head version but if you must have two heads they make the X37 with two heads.
A couple other boats in this price range that you might want to consider:
Hanse 371/370. I have sailed on one of these and find them to be really nice boats all around. Nicely built and well thought through.
Beneteau 40.7: I have raced on one of these for the past 5 years. They seem to be very well constructed (better than the number series boats in a lot of ways) and sail better than any of the other boats on this list with the possible exception of the X-yacht. On the other hand they have less of a cruising interior than the others.
Elan 37: (Elan Yachts) This is a company that I was not familiar with until a couple years ago. Since then I have been board a number of their different models and have been quite impressed. The 37 looks like a really neat boat all around.