I use the TS-E 17mm, and it's by far my favorite wideangle, I used to have the 14 II, but the converging lines limited it soo much for me. I thought the 17, especially after reading all the tech-stuff around, that it would be very challenging to use properly, but nothing could be further from the truth, it's supereasy to use and gives AWESOME results. IQ-wise it's one of the best, only surpassed by the TS-E 24 II, BUT I didn't get it, because I needed wider and also already own the 24 f1,4 II.
I would go for the 17, no doubt, it will make your images stand out like no camera-upgrade can do.