These are both good weapons, I have used them both, and though they are different in many ways, they both do the job.
The Thompson is the epitome of the sub-gun maker’s craft, the units produced by Colt were amazing works of mechanical art. The military versions were not as pretty, and lacked one internal part that didnt really have to be there, and it was a respected weapon to all who knew it. (on both sides) It had a higher rate of fire, and larger magazine capacities, up to 100 rds. The Thompson was as accurate as any carbine of its time. The only drawbacks were cost, and manufacturing time.
The M-3, and later versions, were cheap, fast, and dirty, area coverage weapons, having 6 machined parts, the rest were stampings,and welded together. Cost per unit was around $9.00 at the time. Tolerances were just a dream, some barrels were loose enough to allow the slug to slide through, and fall out the muzzle.that wasnt an issue though, as it was intended for confined spaces, or repelling a close attack. (within about 200 ft.)
the M-3 was available in .45 a.c.p., or 9m.m., the change over requiring a different bolt, barrel, recoil springs, and magazine. I didnt care for the lower rate of fire, but with enough of them going ,it wasnt a real problem. I liked them both, and would enjoy using them again. - Raspenau -