I've had the 50 1.4, the 50 1.8 II, and I've currently got the 50 f2.5 macro and the Sigma 50 EX (pre-Art). Haven't tried the 50 STM, and probably won't because between those last two I'm covered at 50mm just now.
If I was in the OP's shoes right now, I'd give the 50 1.4 a miss and either just get the new 50 stm, or play the Sigma 50 EX lottery. Here's why:
My experience of the 50 1.4 was that while it did bring some limited optical improvement over the old 1.8 II, I could never really trust it. On some subjects in certain light conditions, the AF could miss wildly on my 7D, 5DII and even 5DIII. Even doubling up on shots wasn't enough to guarantee an acceptable result in all cases. On top of that, I rarely liked its bokeh; too busy to my eyes, not unlike the much cheaper 50 1.8 II. And then there was the worry that the legendarily fragile focusing mechanism could let me down. All in all, just not good enough to justify the outlay.
On the other hand, the 50 STM seems to be a pretty safe bet. Most reports indicate reliable, consistent focusing except in low light, and it came out well in a look at copy variation by LensRentals (don't have the link, sorry). OK, it's not brilliant optically, but then neither is the 50 1.4, and the STM is a lot cheaper.
Then of course there's the Sigma option. The copy I've got now is reliable for static subjects on my 5DIII (bit of a lottery when tracking movement though) but crucially I really, really like it optically. I find it to be perfectly usable at 1.4 and the bokeh - especially at f2 and onwards - is really easy on the eye. BUT.. this was my third (or 4th? can't remember) attempt at getting one that focuses well, and I seldom use it on my older 5DII because those two just don't get on, no matter how I adjust the AFMA.
So to summarize, I'd say just get the STM now, and if another lens comes out that doesn't have any worrying flaws (e.g. updated 50mm f1.4 from Canon maybe?) the OP can upgrade without losing much in the process.