I have used the 1.4 extensively, and currently have the STM.
I would go with the STM all day. the 1.4 was a bit soft wide open. and the STM is razor sharp. I love it. for 115 dollars, there isn't a better lens on the market for that price range.
I find that the AF is spot on every time. at 1.8, its awesome. I actually use it in conjunction with a 85 1.2 L and have no hesitations switching to it when needing something a bit wider.
go with the STM...