They look the same but wear very differently.
6mm stuff is rock bottom cheap. For a house the better is 8mm.
The problem for the thinner stuff is the levelness of the floor underneath,
not how solid it is. If there is any dips in the floor, the thinner stuff will bend a lot
and break the joints. The thicker floor can cope better.
Having said that, you should ensure the floor is as smooth as possible.
Check for dips with a long straight edge. Don't just blast it down in an hour.
If you need to, use levelling compound or cardboard to even the dips.
If its for bedrooms, you can go for the lower end of the range.
Downstairs flooring will stand you better if you spend money on the good stuff.
I can buy 6mm laminate for €5 a square metre. Small bedroom grade.
The good 8mm starts around €12 a square metre.
It's hard not to buy the thinner but it is a false economy if you are putting it in
an area where it will get wear and tear.