These models are used in the CCIE training. So the difference between them is one 1 FastEthernet port more on 2621XM and of course the price is a litthe higher.

So buy a 2620XM and if you want an extra FE just buy another module but check the prices first.

I found on cisco page the next datasheet: http://www.cisco.com/c/en/us/products/collateral/routers/2600-series-multiservice-platforms/Cisco 2600 series Routers Product Data Sheet