Nemo hit the target perfectly. All ISPs over sell their service - my ISP puts about 20-50 people on a single 3 mbps line, I'm rural

sadly.
Cable is a shared circuit from you and any others in your area to the cable company, this can get over saturated. Consider if you live near any high concentration of people aka a college, if so you likely don't want cable.
DSL on the other hand is a direct line you alone get to use. (though this doesn't mean they don't oversell - it just happens further down the line)
Another smart idea now-days would be to ask what kind of LIMITS they will hold you to, and if possible tape it too. To this day I still get pissed off about that. When our rural ISP did our install I made it a point to ask if we were going to be held to a monthly bandwidth limit or not - THEY SAID NO, UNLIMITED! Guess what, first week of service and they cut it off. I had to call in and they whined about me downloading 10gb in a week. Long story short, get it in paper if you're a large consumer. Cable in particular is starting to push for metering bandwidth.