From your experience, is the vertical slots on the left and right useful in terms of opening and closing the window, or makes no difference? What colour did you choose? I am leaning towards white so that not obvious when dusty, but worried will be obvious when rust creeps in.

Yes. It is neater compared to without the right and left opening.
If u use horizontal bar design, the opening need to be exactly around the window handle. It will be difficult to open if it is not.

As for colour last time I choose white but it gets rusty over time.
If there is no concerns on budget. Can get stainless steel.

On the other hand , maybe can try stainless steel mess grill ?

what do you do with the rust? paint it over? SS out of budget already.

Ya. Sand it then paint it over