Picture

I did mine horizontal and it pleases the eye better.

My hardwood is diagonal in the living room, parallel to the long wall in the adjoining hall and dining room.

Your installer in this case, for your situation, is correct. The light will shine through your windows and across the floor, and if the laminate extends across the width, the light will highlight every irregularity in the seaming of the laminate. Run it lengthwise.

This info should be beneficial.

Also I'd suggest selecting a laminate with some texture and dimension so the final installation doesn't look like a sheet of plastic. Don't skimp on the quality of the underlayment to deaden the tinny sound some laminate floors have.

Here is a link that might be useful: How to Install Laminate Flooring

I did horizontal as per my installers advice..love it...