Finally found the URL that lists 2106 vs 5508 controller

What are the features that are not supported on the 2100 Series Wireless LAN Controllers (WLCs)?

A. These hardware features are not supported on 2100 Series Controllers:

• Service port (separate out-of-band management 10/100-Mb/s Ethernet interface)

These software features are not supported on 2100 Series Controllers:

• VPN termination (such as IPsec and L2TP)

• Termination of guest controller tunnels (origination of guest controller tunnels is supported)

• External web authentication web server list

• Layer 2 LWAPP

• Spanning tree

• Port mirroring

• Cranite

• Fortress

• AppleTalk

• QoS per-user bandwidth contracts

• IPv6 pass-through

• Link aggregation (LAG)

• Multicast unicast mode

• Wired Guest Access

From:
http://www.cisco.com/en/US/products/ps6366/products_qanda_item09186a00808b4c61.shtml