I recommend multimode. I used to work with fiber on a university campus, and almost all of our fiber was multimode. Singlemode is mostly useful when you need extremely long distances, and unless your building is Boeing's manufacturing plant, you won't need to worry about that.
From wikipedia:
The equipment used for communications over multi-mode optical fiber is less expensive than that for single-mode optical fiber.[1] Typical transmission speed and distance limits are 100 Mbit/s for distances up to 2 km (100BASE-FX), 1 Gbit/s to 220–550 m (1000BASE-SX), and 10 Gbit/s to 300 m (10GBASE-SR).
Equipment for single mode fiber is more expensive than equipment for multi-mode optical fiber, but the single mode fiber itself is usually cheaper in bulk. In 2005, data rates of up to 10 gigabits per second were possible at distances of over 80 km (50 mi) with commercially available transceivers (Xenpak). By using optical amplifiers and dispersion-compensating devices, state-of-the-art DWDM optical systems can span thousands of kilometers at 10 Gbit/s, and several hundred kilometers at 40 Gbit/s.
