Scott pretty much hit the nail on the head. My experience has also been that the dishes in the tasting room are more hit and miss, whereas the ones in the restaurant, by dint of the more limited selection (four of five choices per course), seem to more often hit the spot. But both are great experiences. But if I had one trip, I'd take the main dining room.
Tasting Room: 10 course menu (with optional foie gras supplement) with or without wine pairings. Small room, usually quiet, with comfortable chairs. By way of the format and room arrangement, the focus of the restaurant experience is on the food and wine.
Main dining rooms: Prix fixe with options of 2, 3, or 4 courses selected by the customer. Portions are larger than in the Tasting Room. Food style and quality (with the same chef over both rooms at this point) are comparable. Nice neighborhood restaurant feel, with a somewhat higher noise level (though almost never objectionably so).