I have both for my 6D. However, if I were shooting crop, I'd pick up a Sigma 30, and sell the 50/1.4 to fund a 40STM.
I use the 50mm f/1.4 when I need the low light for events where I can't use a flash, or I want extreme depth of field.
However, at almost every other point, I've got the 40STM on my camera. I like the slightly wider focal length. It's also incredibly sharp. If I'm shooting an event with a strobe, I'll go with the 40STM over the 50mm every time, set it to f/4 and you've got IMO the right amount of DOF, and super sharp images.
I also like that if I want to travel light and without my camera bag, I can throw the 40STM in a pocket, and keep the 70-200 f/4L IS on the 6D. It covers most of the range I need, and I don't need to worry about bag checks or the weight.
Also, another nice bonus is that the 40STM is cheap enough I don't even bother with a lens cap for it, so it's always ready.
Finally, despite thinking that us Canon owners are the only ones with a "body cap" lens, Olympus M43 users have an even smaller option! http://www.bhphotovideo.com/c/product/893289-REG/Olympus_v325010bw000_15mm_F8_0_Lens_On.html
Only downside is it's a fixed f/8 with the focus set permanently hyperfocal.