You might want to consider what color belt you will wear more often, and what color shoes you will wear.

I figure the sporran strap and sporran should follow the convention of matching your belt to your shoes. Just seems like the natural thing to do.

As to which one to buy? Both would be the best!

Another option to consider might be that a black sporran strap used with a brown sporran, while not the best combination, would seem to be a better combination than a brown strap with a black sporran if in a pinch.