My dining experiences at Bistro are all prior to this year, when this restaurant joined the dining plan.
I've dined at both several times. Bistro was very nice, upscale, small, intimate, slow paced, some of the longest meals I've had at WDW. The waiters were in tuxedos and the restaurant always felt quite formal. Food was good, service was very good, wine options were great. Tables were close but there wasn't that many of them so it was always very quiet. This used to be our last night dinner every vacation up through 2010.
Chefs is casual; the food is rustic French so quite different than what Bistro offers. Tables are close; the restaurant is busy and noisy. Service is more rushed and spotty as the servers have a lot more tables to deal with here. I think the food is okay but not as unique as it was prior to the dining plan. I prefer lunch here rather than dinner.