Your stats, level, armor, commendations, that's all saved on the Bungie servers and will be okay. I'm not sure if you have to be signed up at bungie.net ,but the only thing you'd really lose is local files and campaign save data.
Anything that is actually stored on your memory device you will lose, therefore you need to transfer via the Transfer cable, or a standard USB flashdrive. You don't need to be signed up on bungie.net, because all of your stats, armor, etc. is tied to your
XBL account, not your bungie.net account, but it wouldn't hurt to sign up and get it linked to your XBL account, plus it adds a nice "nameplate" next to your name in the game.