Common now to have combo, but have you thought about separate devices? If you had to change to a newer router later on, you can do it. Cable and DSL modems don't change very much, but standard for routers seem to be changing more often. "N" today and who knows, "Y" tomorrow whose signal can penetrate the "gold vault" at Knox. You do have more wires to contend with. From an aesthetic point, combo looks a lot neater. Nevertheless, I would def go "N".
You're asking for the best. Do you mean the meanest and most expensive combo you can buy? That shouldn't too difficult to find. Personally, I think if you can find a device that fits your needs, buy that product. No need for overkill, 'cuz as you know, technology is moving quickly. Save your money so you can buy newer stuff in a couple of years. Fund your retirement instead so that people like the Madoff and Stanford of the world can enjoy themselves now. Sorry, I digress. Anyway, think of how many of your friends wanted to buy the latest tech or get the top of the line computer because it lets you do everything only to use it as paperweight in a couple of years. Hmmmm, pet peeve.
Lastly, if your leasing your modem from your provider, by all means, buy your own stuff.