As others stated, you should decide based on focal length only. Personally, I picked the TS-E 24L II as my preferred focal length, after spending some time shooting my intended TS-E subjects with at both 17mm and 24mm (testing was actually done with the 7D + 10-22mm; I bought the 16-35L II and sold the 10-22mm about a month after getting the TS-E 24L II).I was talking mostly about functionality. You can crop a little but cannot always go backwards.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.
Better? If you're referring to the original TS-E 24L, maybe - but the TS-E 24L II is sharper than the bare TS-E 17mm, and decidedly sharper than the 17 + 1.4x (very obvious in the TDP crop comparison).
Sorry - your statement about image quality suggested otherwise…
That is my quote "as good or better"... I read a review BEFORE I bought my lens (I just looked for 20min and cannot find the review...but I did read it) and there were sample photos showing less color fringing with the 17mm TSE II with 1.4X compared to the 24mm TSE. I was in shock. In any event ...I have shot images with the 17mm TSE II alone and with the 1.4X on the lens. I am very critical about image quality...I have to say...unless you are a professional Architectural Photographer and use the lens daily...or desperately need the f/3.5 for some reason or the slight extra swing/tilt..I cannot see buying both lenses. The results are really SURPRISINGLY fantastic with the converter on. It is something I would never even considered doing until I read the article.
So, raise your eyebrows if you want ...but it is a great setup.