While on casual status at Fort Benning between jump school and pathfinder school I was detailed to the post armory and spent a few weeks detail stripping and cleaning Thompsons, as well as M1911A-1 pistols and M60 machineguns. Got so familiar with those that I could still probably service any of them in the dark to this day.
During my service in Vietnam I had the opportunity to handle and use several vintage pieces. M1 Thompson (WW2 manufacture, probably part of our aid to Uncle Ho while his forces were battling the Japanese), Swedish K-model Carl Gustav (probably brought in by US Special Forces), Chinese version of the PPSH. I once saw a Belgian Browning Hi Power with factory selector switch. Some US tank crews were armed with the old M3 .45 caliber submachineguns, which were generally considered to be junk, but I found them to be 100% reliable although limited to very short ranges for any accuracy to be had.
Left the Army and went on the police department. In the armory we had 6 Thompson M1928's, all Colt manufacture, that had been there since before WW2. Sequentially numbered guns, so I assume purchased from the factory. Being the only one there who had ever actually used the Thompson, I was assigned to strip, inspect, test-fire and clean them. All the ammo on hand was WW1 surplus (headstamped 1918, in 20-round boxes with cloth tear strip) and WW2 surplus (steel cased, 1944 headstamps). I probably burned up a fortune in collector-grade ammo doing the test firing. During the latter 1970's the Thompsons were traded in on Colt M16's (I think the department got $600 each in trade allowance).
Served a few months on a joint narcotics task force that was equipped with Ingram MAC-10's. Extremely effective pieces, especially with the suppressor mounted.
Also during the late 1970's I had a friend needing to raise some cash for a business deal, and bought his M1 Thompson for $550. My first experience with the Treasury's registration process and I remember being surprised when the papers came back with an actual $200 stamp pasted on (took about 30 days to get the paperwork back in those days).
During the early 1980's S&W had a contract overrun sale on their M76 9mm subguns, picked one up on a letterhead purchase for $600 complete with soft case and 6 magazines. The M76 was remarkably similar to the Carl Gustav K-model, and I think there was a little copying done there.
About the same time Colt was offering M16A1's on department purchase orders for about $600 each.
Knew an officer in another department who purchased one of the .22 caliber SMG's, used a drum magazine that held (as I recall) 177 rounds of .22LR. Full auto with one of the earliest laser sights on the market. Name brand and model designation escapes me now, but as I recall these were marketed primarily for prison use, very accurate and controlled fire with small caliber ammo at medium ranges.
The late 1980's brought a sales rep to my office pushing H&K MP5D Navy Models (suppressed, contract overruns from their sales to the US Navy). I recall that the law enforcement agency price at the time was about $500 or $550 NIB with full warranty. Several agencies sent people out to the range for demonstration firing (impressive guns). I had no budget for such toys so I had to pass (after spending an afternoon burning up the free ammo).
About 1990 to 1992 Century International Arms was selling (to law enforcement agencies only) a bunch of used M1 Thompsons for $250 each, Swedish K "Port Said" models for $200 each, and M3 greaseguns were available for around $150 apiece. At the same time the US Department of Defense was authorized to send M1911A-1's, M16's, and M14's to police departments on indefinite loans at no cost (tempting, but I never took them up on it).
So, I have a little bit of experience with a fairly broad selection. Hard to make a definitive choice for only one. For pure "sexy" and historic value the 1928 Thompson wins hands down. For absolute functionality and "best for purpose" choice the H&K would probably get the nod.
I find it interesting to note that about the only real difference the ban on registering newly manufactured NFA guns made was to drive the prices of pre-ban pieces up into the "new car" price range, when prior to the ban they were just about all under a thousand bucks, and relatively available subject to federal and state laws.