I'd also recommend the taxi trip. You'll likely be coming into Terminal 3, but could be Terminal 2 depending on airline. If Terminal 3, after you get luggage and go through Customs and into the arrival hall, follow the signs pointing downstairs to lowest level (use elevator or escalator/ramp) for Taxis. Ignore any touts that approach you. When you get outside on the sidewalk, you'll see the taxi queue(s), there will be a worker directing at the head of each queue, assigning people to cabs. At that time of evening, there may be quite a few flights coming in, but even if the queues look long, they move fast. If coming into Terminal 2, once you get into arrival hall, just look for exit door (same level and nearby) with sign for "Taxis" and you'll see the queues outside. Show the driver the printout with hotel address in Chinese.

Price: Will be metered (I've never yet had a cabbie at the airport fail to turn on the meter). There will be a toll booth charge of RMB 10 which the cabbie will pay, and add to your metered fare at end of trip. Also an RMB 1 fuel surcharge. Have some RMB handy before you leave the arrival hall and head out of the terminal. ATM's are available, or exchange a little cash for RMB at one of the banking counters. If your hotel is near the CCTV building ("Big Underpants") in the Central Business District on the East side of Beijing, total fare including the above surcharges will be in the RMB 80-90 Range. If near the CCTV TOWER on the West side, it will be more like RMB 120-ish. There are no extra surcharges for luggage or anything else. Unlike many countries, cabbies in China do not expect or receive tips. Although for airport trips, if my total fare including surcharge is within 1-2 RMB of a round number like 90 or 100, I generally tell him to keep the change.