I'm not sure how I feel about plastic body and mount lenses like the 50mm f1.8 STM. I had a 50mm f1.8 II that fell apart after 5 years of hard use. The entire focusing tube and objective lens group literally fell out of the lens while I was holding it. On the other hand, it was a very inexpensive way to get hooked on fast prime lenses. I probably got my money's worth. I've since replaced it with the 50mm f1.4 USM.
The poorly-constructed 50mm f/1.8 II left many opportunities for upgrades and the 50mm f/1.8 STM addressed a substantial number of them. Here is a list of some differences between these lenses:
7 rounded aperture blades vs. 5 non-rounded (no more pentagonal bokeh)
Metal lens mount vs. plastic
Improved overall build quality (my original 50mm f/1.8 II broke in half for an unknown reason)
Super Spectra Coating
A much improved manual focus ring
STM vs. Micro Motor (smoother and quieter)
FTM (Full Time Manual) focusing (as long as shutter release is half-pressed)
13.8" (350mm) MFD (Minimum Focus Distance) vs. 17.7" (450mm)
0.21x MM (Maximum Magnification) vs. 0.15x
49mm vs. 52mm filter size (though not much of an advantage from my perspective)
Narrower f/22 aperture available vs. f/16
Accepts normal bayonet-mount lens hood vs. threaded adapter
review from a photographer
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