If I understand correctly, the best BHP's are the T series - from '64-'69. So where do the 70's models fit into the scheme of things? The good, the bad, or the ugly?
It depends on what you mean by the "best". The T-series are some of the best-looking Hi-Powers. The finish is gorgeous and the attention to detail is better than it is on the more modern guns in my opinion. The T-series guns come with a very nice stock trigger in most cases and don't require trigger work usually. Since they don't have an internal firing pin safety, the trigger reset distance is shorter as well. They also tend to have a higher value to collectors who look for Belgian guns.
Having said that, the T-series have some deficiencies as a defensive gun when compared to a new MkIII. The T-series has a tiny safety that is difficult to use, smaller sights that aren't as easy to pick up, the older humped feed ramp that doesn't feed hollowpoints well, a slightly smaller ejection port, and a hammer that is more prone to biting the user. All of these can be fixed with aftermarket parts and gunsmithing but then you are spending more money to decrease the collector's value of the gun.
If I wanted a Hi-Power to enjoy and shoot recreationally on occasion I would definitely go with a T-Series. If I wanted a Hi-Power for any kind of defensive use, I would go with a newer MkII or MkIII model.
The MkII corrects most of the deficiencies of the T-Series by enlarging the ejection port slightly to insure better reliability, adding a larger ambidextrous safety, high-visibility sights, a straight feed ramp that feeds hollow points and a spur hammer that isn't as prone to biting.
The only two cons I can think of with the MkIIs are also pros depending on your point of view. The MkII has no internal firing pin safety - which I like because of the short trigger reset - but it can lead to an accidental discharge if you don't replace the firing pin spring when you replace the recoil spring. Even then, the gun can fire when dropped if it develops enough inertia (unlikely with a firing pin spring in good condition). The other con (or pro) is that the MkII is a forged frame.
The MkIII is an improvement on the MkII and adds an internal firing pin safety that physically blocks the firing pin but also makes for a longer trigger reset. After 1993, the frames are also cast instead of forged. The Israelis used an odd combination of MkII slides and MkIII frames on their Hi-Powers.
Both the MkII and the MkIIIs generally have noticeably worse stock triggers than the older T-series and will probably require some tinkering to get it to where you like it.
I'd second Broken Arrow's comments. If you can, you certainly should try and shoot both guns before making a decision and go with the one that suits you best.
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Edited:
Just noticed that I had prattled on without answering the question.
So where do the 70's models fit into the scheme of things?
They are a lot more similar to the T-series than the MkII or MkIII and will have most of the same deficiencies and strengths as a T-Series. Browning made the switch from the older style round hammer to a spur hammer sometime between 1970 and 1973 (can't remember exact date) so if there is any major difference between the 1971 and 1972 Hi-Powers and a T-series, it would be the new spur hammer.