The SB5120 is NOT a wireless access point. It is simply a newer model designed to support as many as 32 individual connections (external IP addresses to separate PCs) on the network. If you use a broadband router to act as a DHCP server on your network, then this feature doesn't do anything for you.
If your ISP supports both modems, I don't see any reason to spend $20 more for the SB5120 over the SB5100. Both support DOCSIS 2.0 and have 38Mbps maximum download and 30Mbps maximum upload speed capacity and have USB and Ethernet connections. (I do have the 5120 since that's what Comcast gave me and it works great, but unless you need multiple PCs with separate public IP addresses, I don't think it will make any difference on a home connection.)
Edit: One thing that IS different is that, at least at NewEgg, you can do a direct replacement for the 5120 with NewEgg if the modem is defective, but you'll have to go through Motorola to get a replacement for the 5100...