I have both setups, but I prefer having the separate keyboard. I don't usually take it with me when I'm on the go. But at home whenever I need to type more than a couple sentences, I use the keyboard. The keyboard case is just too bulky and not worth carrying around, for how little I need the keyboard on the go.
Ditto the comments on the Zagg keyboard being good. I have the Zagg keyboard that comes with its own case that also can be used as an Ipad stand, which is nice. The cut and paste keys are extremely convenient. Also, the buttons to control your music are pretty cool, too.