Cable connections, just like DSL, vary from area to area. RoadRunner in Charlotte, NC is different from RoadRunner in Columbia, SC and RoadRunner in Rochester, NY. It's even different in different areas in Charlotte itself. It all depends on the equipment in the area. Check around with your neighbors, and see if they have it, and what they think.
Earthlink highspeed goes through other providers. While they might have service in your area, they're most likely going over Comcast equipment and cable lines. Their "real" service is all the extra addons.
As far as the DSL vs. Cable debate goes, like I said, it depends on the area. I've talked to customers who have had better luck with DSL, and customers who have better luck with Cable. Since I work mostly with telcos, I get a lot of DSL calls... the ones that are always on seem to be better than the ones that are PPPoE (you have to actually connect to the internet, like dial-up, but still much faster). Then again, the same goes for cable. Yes, there are cable modems who dial-up, they're called one-way cable modems, or telco return, the upstream going over a regular 56k dial-up line. And those well and truly suck. blech.
