The EF 50mm f/1.8 STM is generally the better choice.

According to the community answers, the two lenses are very similar optically, with the STM version at least matching and in some comparisons slightly improving sharpness from about f/1.8 to f/8. The bigger differences are in handling and features:

• 7 rounded aperture blades vs 5 on the II, for smoother out-of-focus highlights
• Metal mount vs plastic mount
• Much better manual focus ring
• STM autofocus is quieter than the older micro motor
• Full-time manual focus
• Closer minimum focus distance (0.35m vs 0.45m)
• Higher maximum magnification (0.21x vs 0.15x)
• Smaller 49mm filter thread
• Minimum aperture of f/22 instead of f/16

So if prices are close, the STM is the easy recommendation. It gives you better build, more refined autofocus, better close-focus ability, and nicer bokeh, while keeping the same basic 50mm f/1.8 value. The older 50mm f/1.8 II only really makes sense if you find it significantly cheaper and don’t mind the simpler build and autofocus.