If you ever use it when your laptop isn't plugged in, it will drain your battery faster. You won't have to carry a power adaptor if you travel. Other than that, there's no real difference. Personally, I find it silly to spend the money to come out with the same size unit just to lose the power supply...
yeah. if you REALLY need that much space on the go, i would recommend just putting a bigger hdd in your laptop. if you need hella storage on the go, get a slim hdd. you wont be able to get 1tb...but that's kind of excessive for being mobile anyway.