I really appreciated the service at Madewell vs. J Crew. Two SAs helped, one with the trench and one with jeans. Both went above and beyond (checking on sizes and water resistance status) and were knowledgeable about all the stores products, how they wear, the fabric contents, etc. Whenever I've gone to Gap, they've known nothing. J Crew was helpful, but just average IMO.
As far as quality, I fell in love with a few different items while I was at Madewell. A citron top and a citron prism sweater. They both seemed like items I could easily find at J. Crew (though the prices would also be found at J. Crew). It is definitely a more casual store - but for many things this was a good thing. More items seemed to resist wrinkles at Madewell, at least, like the dresses I touched and the wideleg, linen-like jeans I tried on.