There's pros and cons to both but I personally prefer the Smart Keyboard option. It's lightweight, I find it excellent for typing on and it's always there without having to carry a separate keyboard.
Plus there's the advantage of being able to easily and comfortably type away using the Smart Keyboard on your lap, not so good with the other combination.
Everyone has their preferences but to me the Smart Keyboard is amongst the best keyboard options for an iPad I've ever used. I'm not fussed about backlit keys, the screen provides enough light to see in low light situations
Not particularly bothered about the lack of dedicated function keys, the keyboard shortcuts do fine for most things and the couple it doesn't, well it's hardly a hardship to touch the screen considering it's right in front of me.