Does it matter which modem is used? I am using a Time Machine connected to an old-ish Motorola cable/broadband modem (SB4200). I have been paying rent for the modem for a number of years and have more than paid for the purchase cost. I want to eliminate the monthly rental cost so have figured I will purchase a modem. Which works best and why? I don't see anything that says one is faster than another or that one is more secure than another. My assumption is that it is just an access point and any security I have is built into the Time Machine. Is this correct?