I think that if the Tommy Gun fed hollowpoints reliably and you had one of those 100 round drum mags, the Tommy would be better at close range. Remember, the .45 is a pistol cartridge and the range is limited whereas the AR uses a rifle round and has greater range and accuracy when you get out there.
Either one is lethal to the extreme, but the AR is a better choice. The AR has an edge in weight and handling, and has better performance against body armor if that matters. Those 100 round mags add too much weight and little gain in capability. If after 30 rounds the lead is still flying, you need to be finding a hole to crawl into anyway.