Hi Adrian,
Some answers to your queries here:
I don't think the earlier 944 had a narrower cam belt, but they did have a narrower (15mm) balance belt. The latest one is 18mm and I think fits all cars.
The spring tensioner is locked down after it has applied a preload to the belt. It doesn't do anything further after that, so once a non-spring tensioner has been adjusted right, they both operate in basically the same way. Neither will stop a belt snapping or jumping at idle.
The S2 has the same cam tensioner as the S and 8-valve from 87 onwards. Only the 968 had a different oil pressure operated one.
The 944S is worth looking at, I have had a few of these and like them. It's all down to what you expect from it and how you drive! Under 4000 RPM it's basically the same as the 8-valve (some would say not as torquey), but once past 4000 it comes 'on song' and feels great, so just drive it hard with plenty of revs - I do sometimes...it's fun!
The 2.7 was an increase in the 8-valve engine, I do not think it applied to the 16V S, as the 3.0 S2 had been released by then and the S discontinued.
With all cam chain driven heads, yes you can hear the chain, but generally the hydraulic tappets (lifters) drown it out!

Always just get the chain and tensioner pads changed, along with the cam belt, etc. if you don't have an up-to-date service history that states this has been done.
Model preference is always down to personal choice, so go out and drive a few...
Look forward to seeing you on the forum once you make that purchase!