Ps90 flash hider, which one and why?

Comp, because I view the PS90 as more toy than tool, and flame is fun.

One is first gen, the other is second gen. The newer one works better but is only available as an expensive FN part, while the first gen has been cloned by a few different third parties and can be had for much cheaper.

Sadly neither allows you to mount a can, so the third option of a threaded adapter is usually the one most folks go with. The threaded adapter is usually cheaper than either of the permanent flash hiders and it allows you to mount a suppressor or a 5.56 flash hider of your choosing. It won't look "classic" though.

I like the look of the bottom one piece flash hider.

But I plan on using a suppressor so I'll be going with a thread adaptor and a specwar mount.

Get the flash hider that is compatible with your suppressor, or vice versa.