The 16- is a budget rifle. This sounds like an insult but back then budget meant something else... you got plain wood and basic sights.
If you are into that, the older years had a steel trigger guard.
They have a CRF action, as smooth as a CRF action can be, but triggers are usually pretty bad compared to more recent designs.
The bolt release lever is weak and fragile, and has a tendency to stick when dirty, allowing the bolt to leave the rifle if pulled fast enough. You must keep it clean.
The stocks crack often with age because of the design of the rear action screw. Proper relieving and bedding eliminates this.
The 1900 is completely different and is what I would call a higher quality product, even if it was designed with some cost cutting measures.
It usually has better wood and bluing, and triggers are as good as anything else, and are adjustable.
I prefer them to the contemporary Sako offerings but they are comparable in quality. The bolt is as smooth as a well used T3 and has 2 lugs and a Sako style extractor. The simple safety locks the bolt closed. Pretty basic; but well executed and refined.
I have to say they feel heavier than the 1640.
I had the chance to compare all variants of 1900 side by side when visiting someone who has more of them than me: Husqvarna, FFV, Carl Gustaf, unlicensed Zoli and licenced Zoli. I couldn't see a difference in quality, but there were small differences in stock and sight design. The Antonio Zoli 1900 is still made today and is a fine rifle.
If you are shopping for these you may also be cross shopping the Carl Gustaf 2000, the finest rifle of the 3. It is as fine a rifle as anything you can get under 3k and is nicer than contemporary Sakos, except deluxe models.
The controls are pretty much like a Sako 85 and allow unlocking the bolt when on safe. Very good triggers.
It has a very short bolt throw that leaves the bolt handle parallel with the ground, due to a 3 lug action that also has a bolt face that surrounds the cartridge completely, the Remington 700's best feature. It has a 45 degree main action screw like a Ruger M77.
The 1900 and 2000 have no cons that I know of.