They are laid 90 degrees to the joists for the greatest stability.

sorry, forgot to mention they are being laid onto a concrete slab. I think glued down but i am not sure. I don't think it is a floating floor as they plan to use leveling "stuff" first.

I would run the floor with the length of the room; running across the narrow width will give a 'ladder rung' effect, which usually is not visually appealing.

Have some flooring layed out dry both ways and then decide which looks best to you.

ANY flooring project requires a smooth and flat substrate; that is why substrates are flattened prior to any quality installation.

Generally running the floor the long way of the room make the room look wider. Consider a hall that's 3' x 16'. Running the floor the short way will make the hall look even longer and narrower.
A someone above pointed out, wood was run perpendicular to the joists and the joists generally ran the short way so that's the tradition.
You must also consider how the floor will look as you enter from various doorways. In a room that's 25' long you could have two furniture arrangements with a strip of uncovered flooring running between two doorways, so something that might traditionally seem incorrect might look best.

What about running the boards diagonally? Can someone comment on that choice...I have heard it can open up a room visually even if it's already a good size room.

Running on a diagonal can visually stretch a space.