If you aren't planning on wearing a lot of gray suits, I'd say brown over navy. I wear navy blazers at least twice a week, and if we include blue, probably four times a week. As a result, I rarely find myself reaching for ties in the blue range. The occasional gray suit, though, always gets paired with a blue tie. So yeah, if you wear a lot of gray, a navy tie is a priority. If not, you'll get more out of brown and burgundy.