Since you already have the brown/patterned sport coat, I would get a solid one for the 2nd choice. Like others said, navy is good...also, grey/charcoal solids are very versatile as well.
what's the best color pants to go with dark brown sportcoat?
Pretty much all shades of tan/beige, and for a little more dressy look grey shades. One color I like using a lot is white or off-white.
If the dark brown sportcoat has a pattern, sometimes it`s cool to match the pants with the color of the pattern too. For example a dark brown jacket with moss green pattern...could go with moss green pants (It does`t work like this all the time, but it can).