The Thompson was much faster and easier to control.
My late father trained on one in the Army. Said it was very controllable with its slow rate of fire and straight back stock.
Key to making it shoot straight was NOT to hold the wire stock firmly against you shoulder but sort of let it bounce off as the bolt moved forward. The recoil would bounce it back and you could walk the fire into any target is a short time.
Never fired the M3 but did fire a M1A1 made by savage. frankly I did far better with hits at 25 M on repetition setting than on the automatic setting. The Thompson has excessive drop in the stock and with the higher rate of fire, well it seemed to have excessive vertical dispersion. On the repetition setting, with that slight stabilizing influence of time, you would get more hits past 25 M than with full auto on a 20 W by 30 H pistol target.