My grandpa favored the grease gun much more than the thompson because it had a more accuracy when he used them in WW2.
Dad started his Army career as a tanker in the mid-50s, and he loved the grease gun. Went on to be a pilot, but spent some time in M41s.
The grease gun was lighter and fired slower. Perhaps that was why it was more liked and more accurate. I carried both in Nam. ( along with an M-79, M14, M16, 45, etc.
As a former Army Armor Center M3 and M3A1 *grease gun* instructor who carried the things as a young tank crewman and later used one in a shootin' contest in which the targets shot back, I can tell you this about the M3 versus the M1A1 Thompson, which I've also used extensively: it's about 4 pounds or two loaded magazines worth lighter. The magazines for a Thompson are a little shorter and more compact, a consideration if you carry spare magazines stuffed down the front of a set of tanker coveralls in hot weather. Magazine pouches hang up on the edge of a hatch and on interior fittings waaaay to easily.
Both guns are surprisingly accurate, and anyone who's had a little practice with one can ruin the day for anyone within a caty block. Nighttime flash is also about the same.