IMHO, The M3 is a better quality weapon (particularly the M3A1), but it is plagued by poorly designed single column feed magazines that lead to occasional stoppages, and does have a very slow rate of fire (450-500rpm) that some find objectionable, I rather like it.
The STEN is crudely built but quite reliable with good magazines. STEN magazines are well designed, using a double column feed, but the quality of construction varies widely. Figure on buying twice as many mags as you need to get enough good ones. If you have a stoppage when firing the weapon, mark the mag and put it in a questionable pile, if you get 2 stoppages with the same magazine, throw it out (they are so cheap I consider them disposable), or mark it for range use only.