If you're going against a mostly air based army, get Ninjas. They're much cheaper, build fast, and thanks to stealth they get a x2 damage bonus on most shots.
Also, remember to check all the resource locations for expansions and clear banks during down time. You left two undefended refineries alone until they were almost mined out, and you let him control three banks (one right outside your base) that you could have easily cleared with tigers.