Is there a noticable difference? Is 1.2mm ok, or should it be avoided?

I imagine it would come down to the design for the wind loading on the window.

 

both are  like tin foil so Id  go for the 1.5mm all the fittings  they "usually"  fit are crap as  well. I install myself as somchai uses 6  screws  at best, I also pump expanding foam into the massive empty moldings on the frames and put a  frame around them to make them look not so naff using cement board  shera  type  stuff and double the locks

47 minutes ago, gunderhill said:

both are  like tin foil so Id  go for the 1.5mm all the fittings  they "usually"  fit are crap as  well. I install myself as somchai uses 6  screws  at best, I also pump expanding foam into the massive empty moldings on the frames and put a  frame around them to make them look not so naff using cement board  shera  type  stuff and double the locks

Nice

1 hour ago, Farangwithaplan said:

I imagine it would come down to the design for the wind loading on the window.

 

4 pannel sliding, and single sliding

17 minutes ago, banagan said:

Nice

forgot to add the bottom rail is raised up 1  inch or  so to allow  for tiling  later on, thin set adhesive not their crazy 2  inch  plus screed

On 8/15/2019 at 8:13 AM, banagan said:

Is there a noticable difference?

Yes. 1.5mm is thin. 1.2mm is ridiculously thin.

 

But it does all depend on the size of the window/door and the number of cross-members (if any). The larger the opening the thicker the aluminium should be. In practice here you only really have a choice of those two sizes, unless you are prepared to pay fantasy prices for imported frames.