Lacroix or the Fountain?
What a pleasant topic to ponder! My husband have been twice to both in the last year and honestly - both were excellent. I think the food, creativity, presentation, and ambiance are fantastic at Lacroix, and while less creative, per Mr. Pickles, I have never been able to find fault with Fountain. Great food, fantastic service. We tend to go there for our most special occasions (birthday, anniversary) and stick to Lacroix for Valentine's Day, and other special dinners. We have never been to LeBec Fin - just haven't been able to get in on as short of notice as we usually plan - so I don't have a comparison there. IMHO, Vetri was only "okay" when we went - maybe because we were with a 6 person group? I may give that place one more try.
These are both excellent restaurants, although I would rather go to Vetri than either one if I wasn't going to Le Bec-Fin. That being said, Lacroix is just fine even with Jean-Marie's departure from the kitchen. I'd say that the cuisine at Lacroix is more innovative while the service and ambiance are a touch better at Fountain. I kind of like how they handle meat dishes better at Fountain, whereas Lacroix does a slightly better job on seafood. Wine lists are decent at both places. During the summer, Lacroix was BYOB on Monday nights. I'm not sure if they are continuing this policy in the fall and winter. You could always go to Fountain for dinner and have Sunday brunch at Lacroix. My only other comment is that I while I would not hesitate in describing Lacroix as a French restaurant, Fountain has seemed like more of a "New American" restaurant (strong cooking technique rooted in the French tradition, but the soul of the cuisine doesn't strike me as particularly French).