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If you will be installing your routers in an environment where other AC machines will be housed, it is better to use AC power supplies. The simple reason for this is consistency. All devices should use the same power type. Some people are afraid of DC power because there is no receptacle - the power feed gets wired directly to the power supply. This means that you can't simply unplug the router and move it.
Source: Cisco Router Configuration & Troubleshooting by Mark Tripod, New Riders, 1999
To find out more about Cisco Router Configuration & Troubleshooting, go to http://www2.newriders.com/cfm/prod_book.cfm?RecordID=44 and http://www.informit.com/product/0735700249/.