Hard to say, but I wanted to get the TC17III if it ever comes out (to go with my TC14II), so I am holding off myself before I rush into getting anything. I might even get a TC20III, although those are apparently in short supply (haven't checked myself).
They say that the TC-20 III is the first 2x worth owning. Some of the test people were very surprised that it did as well in their tests that it does.
But for me, unless I am shocked and surprised by the TC-17 III's performance? (which I certainly might be and I certainly hope that I am) , I am sticking with the 1.4x.
Hope the rumour is true and they come out soon.