Ho! well, I'm no Guru, at least I hope so. Don't wanna be some kind of Kool-Aid freak a la Jim Jones

- I"m just a very curious guy.
The main differences between the M/94/96/38 variants and the M/98 and the 1640 are;
M/94/96/38 are small ring, ####-on-closing actions with poor escaping gases handling (if a case failure occurs). The bolt have two front lugs and the bolt can back-up in case of major action failure.
But don't get me wrong, they're good actions, as long as they are respected for what they are, means actions designed when cartridges pressures did not exceed 45 000 PSI - the metal used for the se actions is low carbon steel, such as SAE 1025 (but, yes, we all know they can handle more).
M/98 and 1640 are similar (while different) actions, the M/98 being a Large Ring action and the 1640 a Small Ring action and both are ####-on-open actions. Both have better venting of the back of the chamber (in case of case head failure) and a third lug which will avoid the bolt to back-up in case of major action failure. The 1640 is a modern action made of modern alloy, tested following CIP method (but Sweden or HVA were / are not not member of the CIP). The "1600" is in reality the short barreled version of the 1640 (nominal).
All in all, the M/98 and the 1640 are much stronger actions, up to today's Magnum power.
Now, the early 640 series were made on M/38 while later were made on M/98.
The early (and all others M/94/96/38 derivatives) have short magazines, not longer than 3.240"/3.260" for the model 649 (9.3X62).
The M/98 have "standard" magazines, wich can hold 30-06 lenght cases (3.340" OAL).
Zastava are the FN "modernized" M/98 (named #300 and / or Supreme) actions. There is no such things as Zastava M/96.
Most post war M/98 are better than the previous ones, as they were made of alloy steel, not Low carbon steel.
All in all, it will depend on what you are looking for. If you think you may try to drive your bullets at the speed of light, I would suggest you stay with more modern designs. The rest is a matter of personal choice.
Check out the Husqvarna sticky especially in the first pages where the differences between the actions is clearly pictured.