The 17-40 was designed as a wide-angle zoom for film/ff bodies. That is what it is best at. The 17-55 is purpose built as a standard zoom for APS-C bodies. That is what it is best at. The 17-40 is competent as an APS-C standard zoom, but not the best. The 17-55 is better in that capacity. I would recommend to anyone buying a standard zoom lens for an APS-C body to get the 17-55 because it is the best lens for the job.

However, you already own the 17-40. Finding differences in real world IQ between the two might be a task in splitting some fine hairs, especially if you primarily shoot from tripod (thus eliminating the 17-55's IS advantage). For me the biggest issue with the 17-40 as a standard zoom is that 40mm is just too short on the long end for flattering portraiture. But you'll have to make that call based on how and what you shoot.