I got first the TS-E 24 and then the TS-E 17. If I were to start all over I would start with the 17.
I bought the 17mm TSE II (incredible) Lens first...based on the fact that (much to my surprise), I could use my Canon 1.4X III to increase the focal length of the 17mm to 24mm and get as-good (OR BETTER !), IQ as the 24mm TSE.
I already owned the 1.4X III to use with my 70-200mm...and NEVER would have considered using it on a super wide-angle lens...BUT ....I find that the combo is INDEED impressive! Of course you lose an f/stop from the 17mm making the the combo quite a bit slower slower than the 24mm TSE (f/5.6 vs f/3.5 on the 24mm)...but considering all of my work with the lens is done on a tripod and low DOF isn't something that I am usually looking for with a super WA....for me it was like buying two TSE's for the price of one! I am AMAZED at the results with the 1.4X III. It goes against all I have learned over the years...but there it is.
Something to consider when looking at these lenses.
You still have the drawbacks of filter placement, protruding element of the 17mm...it is a little more cumbersome ...but I find it to be worth the money savings for me. If I was a pro, who shot Architecture all of the time, perhaps not.