We are going to buy a Mac Pro (3.2 quad core).
It "needs" 16GB ram.
Is there any difference in quality (type/brand)? Which ram is the best to buy? Which one is better to avoid?
As long as you stick with known brands (Kingston, OCZ, Crucial, G.Skill, Samsung etc...) there shouldn't be any issues. As you're from the Netherlands, I would suggest Crucial. If you're planning to upgrade to +16GB in the future, make sure to buy 8GB modules so you would still have two empty slots with 16GB. They cost a bit more though and cannot be mixed with other modules
I'm not sure if we have the 'time' to get the 8Gb ones. They are still pretty expensive here, but maybe when we will order the Mac Pro they will be affordable. I will keep it in mind. Otherwise we will go for 4 x 4 and if necessary change them all in a couple of years to 4 x 8.