There is a difference. However, hard data is difficult to find. If one travels a lot, they can reach their own conclusion even though even a frequent traveller will have a small sample size.

AA damaged my bag beyond repair and said it was only worth $25. Try finding a $25 even at Wal Mart or a street vendor. Another airline fixed the bag and yet another airline paid $120. However, these incidents span a 20 year period because my bags are rarely damaged. AA's $25 offer can't be explained by inflation.

As far as courtesy, I found that Midwest Express (soon to be renamed Midwest) does have significantly friendlier ticket counter agents and FA's. However, they fly mostly to Kansas City, Milwaukee, and Omaha from a few East Coast cities and Los Angeles (see website for map)

Delta used to be good but not any better than the rest during the past 5 years or so.

US Airways isn't bad, but only a little better than the rest, in my opinion.

UA was ok in the 80's but service has suffered badly in recent years. AA isn't any better.