what's faster from Capital airport to Tianjin train station, train or intercity bus?

Train may be a little faster, but pesonally the intercity bus is more efficient. You shall see the difference:
by bus, the ride is 2.5h to Tianhuan Bus Station. Then you can take a taxi to the train station in around 15min.

by train, you need to take shuttle bus line 10 to Beijing South Railway Station in 70-80min. Then you need to buy the train ticket, check in and board, which may take 15-25min. Take the bullet train to your destination in half an hour then.