Hello. It just sort of depends on what you define as "better". Some of the '60's vintage T-Series Hi Powers (called that because there is a "T" in the serial number) had polished bright blue finishes not seen today. Until the advent of the Mk II, most Hi Powers came with checkered walnut stocks like the ones that come only on the "Standard" now.
On the other hand, the fixed sight pistols had smallish fixed sights that some folks could use at speed but that many Hi Power users wanted to replace with higher visibility sights. The classic Hi Powers more frequently had better trigger pulls than today's guns, at least that's what I remember, but some were poor...quite poor...just as today. Sadly, Hi Power trigger pulls have never been their most endearing feature though I have seen both new manufacture and old classic Hi Powers with much better than expected trigger pulls straight from the factory.
The older guns had a smallish single-side thumb safety. Many preferred either an extended or extended ambidextrous safeties and FN finally obliged with the Mk II in the 1980's and continued with the Mk III, the "chasis" for the Hi Power models seen today.
I like and own both but if I had no Hi Power nary a tall and found an older one that was like new and a NIB Mk III, I'd probably buy the latter. The reason is that I find them just about fine to go from the box once the factory grips are replaced, the hammer spur bobbed, and trigger work done. The same things might very well be required on the older gun, but with the addition of some larger fixed sights. The feed ramp would likely have to be "throated" to remove the hump if I wanted the gun to reliably feed other than FMJ ball rounds. The straight feed ramp first seen on the Mk II eliminates this problem completely...at least it has in my experiences with the Hi Power. (This is for me and my perceived needs; others might very well feel differently.) The Mk III-type Hi Powers we see in the US almost always have the internal firing pin safety and the ejection port has been altered to prevent slide cracking, which could (infrequently) appear in heavily shot classic Hi Powers using really hot ammunition.
I've shot both the older Hi Powers for years and have more than a few rounds through the Mk III and its kin. I prefer the latter for a shooting gun.
This is my first Hi Power. It is an older '71 commercial and started life as a blue adjustable sight Hi Power. It has been customized and has served well and reliably for decades.
Out of the box, I find the current Hi Powers to meet my "needs" out of the box with but a few changes that can usually be done at home.
If a fellow or lady happens to be a real Hi Power fan, they frequently wind up with an example or six of each.
Whichever you wind up with, you will find an exceptional pistol that needs a bit of tweaking to be at its best in my opinion, but one that will serve well.
Best and good luck.