Also another plus is that if the modem breaks or if a newer, better model comes out, all you have to do is call Comcast or whoever and they swap it out. I'm on my third modem in four years. If I had bought it I wouldn't be so quick to upgrade it.
Honestly, for a modem, there really isn't much need for a "newer, better model." The reigning optimal cable modem for anyone without extremely fast cable internet (over 100mbps download speeds), the Arris Surfboard 6141, has been on the market for 4 years now. It's great at what it does, and it's hard to really imagine an "upgrade" being needed unless using a faster connection. (The 6141 costs a little under $70 new and $40 refurbished on Amazon.)
Routers have a shorter upgrade cycle. But, the vastly overpowered router I just bought is close to 2 years old. It's hard to predict the future, but I expect it to remain more router than I need throughout its lifespan. My service provider charges $16/month for modem and router rental (although it's "only" $10 with my current package if I were paying for it). At either rate, I should save a fairly significant sum of money over the life of the products I bought and I get the benefit of a much better router than what I would be getting from TWC.
At the same time, it's worth noting that some promotional deals don't charge for a modem or router at all in the first year. If that's the case, you obviously shouldn't consider purchasing.
Back to advice on WiFi/Internet, the big things to think about are 1) how much speed do you need? and 2) do you want cable and, if so, how much is it worth to you? The first question determines both your provider options and your price point. The second matters because, as others have noted, cable (TV) might not cost that much extra. (I assume you don't value a landline, although if you do, you can probably get it basically for free.) The last thing to keep in mind is that you should assume you'll be paying about $20/month more than the advertised price if you get a package that includes TV. If not, I'm not sure how many fees you'll end up paying, but there will definitely be added cost.