I would stay with NW. The only benefit to AS is that you'll get to fly AA and earn miles. How many times are AA that much cheaper than NW, CO, DL or AS? I'm a CO member and I haven't had to fly anyone other than those 4 all year. Plus with NW, you get great international awards, automatic upgrades on both NW and CO. NW also has great offers from time to time to earn extra miles.