Tony,
I've never owned a 16" AR, but I do have a 14.5" as well as 20" rifle's.
I even have a 18" model.
I think all opinions on this are somewhat subjective.
For iron sights, I would say it has to be a 20" because of the longer sight radius.
The 20" models also have longer handguards so are somewhat more comfortable and easier to shoot. (My opinion).
If you want to hunt with it, then I'd say go for the longer barrel. More velocity and energy.
I believe there is considerable velocity differences between 16 & 20".
If you're planning to shoot longer distances then again it should be the longer barrel.
More velocity = less wind drift.
I'm sure you'd be happy with either choice.
Mark
ps, on another subject, what are silencer laws like in Germany? Are they readily available, easy to get, costs??