Dont know much about the 5100, but I'd take it over my 4100 anyday.
Im a newer customer to Yahoo DSL and received a little black box called a Speedstream 4100. The first one overheated and literally started melting. Modem Number 2, well right after I plugged it in, it started to smoke. Modem number 3, it hasn't over heated or died but to get any speed over 1000kbps I have to log into the modem (common routable gateway of 192.168.0.1) and hit restart modem. So I called yahoo last week and upgraded to the 2wire.
Powercycling the modem doesnt work, unplugging cords doesn't work, it only works when I go in and hit restart inside the modem setup page. I was having to do this 10 - 15 times a day, just to get decent speeds. What can I say, I like to play WoW and hate latency. At first I thought it was the lines to the house (I cant say its the line to the outlet, that runs less than 3 feet and is a dedicated DSL line), but I had those replaced and it still happens.
Well yesterday I received my 2wire Wireless Gateway from Yahoo, in hopes this would cure the problem. I have yet to have to powercycle, reset the modem to obtain good speeds, or do anything other than swap modems.
From the recent experiences with the 4100, I think its crap. Dont mind me, I rant a bit when Im angry at my internet.