You should go with the stem that gets your bars to the right height for your riding position. On that bike, I doubt a negative angle would accomplish this unless you use a really long quill like a Nitto Technomic. You also have aesthetics to consider and my personal opinion is a positive angle stem would look much better on that bike than a negative angle with a lot of exposed quill.

Positive angles were used on mountain bikes to achieve desired bar height on a frame with lower standover height. You wanted lower standover height to better navigate off road.