So you are staying at Tiananmen East Subway Station? I guess.
The train takes about 69 minutes while the express bus takes about more than 50 minutes if there is no traffic jams. No big difference.
And Jishuitan subway station( the nearest one to Dengshengmen to take bus) and Xizhimen Subway station (the one near Beijing North Railway station to take the train) is only one stop distance. No big difference either.
My advice is that, if you'll visit the wall on holidays or weekends, train will be better to aviod being stuck in a trffic jam.