This has been the marketing for both types of service. The DSL guys say that they are faster because the bandwidth is equal all the time, the cable guys say that in the off-peak time (or less-peak time) cable has a much faster throughput. They are both right.

Security not much of an issue either way. Both of them suck. Comcast breaches their own security at will (re: the recent admission from Comcast that they have been spying on you), the DSL guys can do the same.

Sure, I could potentially put a sniffer on the line and pick up packets from my neighborhood. I guess some people do, but there are easier ways to get the data if I really wanted it. My company uses a VPN program that encrypts/decrypts the data over a public network (internet) back to our server. Its more of a pain than having the clear data and speed.