Assuming 1) you do 2 round-trips IAH to LeftCoast per month for 2nd half of year, 2) you earn silver w/ BKK trip , and 3) you expect to fly twice/month next year, I'd return the miles and purchase the tickets. By year end, you'd be Gold, doubling your bonus miles and increasing your chances for upgrades (ie you will have pulled yourself out of the silver muck).