You'll find people that love AT&T, and people that loathe AT&T. You'll find people that love RCN, and people that loathe RCN. The cable modem thing is a crapshoot. My advice would be to go with the one with the best deal. And the less support you're likely to need, the better.

Since you're in Somerville, DSL is probably right out for you. (There's only one CO in Somerville, and they've been switching their phone circuits over to a non-DSL-compatible kind. Pre-qualification won't tell you this; you'll only find out after you've placed an order.)