Hi there. Tricky one this one.
I did most of my preparation for my first half marathon in St Albans earlier on this year on the treadmill with an aim to finish in 1h45m.
Then one day I run outdoors and found it much harder, but, as the weather was nice, I started running outdoors more often. In the end I finished in 1h47m.
Now that the weather isn't so great, I've gone back to the treadmill as personally I don't fancy running in the mud/rain/cold too much. I think a balance of treadmill/outdoors is good. The treadmill can help you pace yourself (especially if combined with a heart rate monitor) and build up the miles durig the winter, but running outdoors will prepare you for the real thing.
Also, if as I do you combine a bit of weight lifting with your running, sometimes it is just easier to jump on the treadmill straight after the weights, it's handy to have everything in one place.
And finally, so sorry for the lenghty diatribe, the treadmill is more mentally fatigating, which is not bad training either if you plan to run long distances.
Take care everyone.