Joe's 1000x600x200 steel and aluminium router started

[JAZZCNC]

I'm also sure I've seem some pictures on here of peoples gantries constructed from plate?

Yes probably seen gantry I've shown built from plate and few others used it or adapted to there machines. It makes for a very stiff gantry,

However, it needs a lot more work compared to HD profile. I've used all methods of constructing gantry's and HD profile is by far simpler with more than enough strength for any Router cutting materials up to Aluminium.

Aluminium Rect section doesn't compare to HD profile in terms of resonance and vibration dampening and this is should be more of a concern than twisting deflection you seem to be chasing if you want a good Router.
There is no cutter you will use that will withstand forces enough to twist a gantry made form any of these materials before it snaps. However, Resonance will affect any cutter and the smaller the cutter the worse it gets. My advice is to Stop chasing twisting etc and look more to lowering resonance if you want a good Router.