Which of the following is more important
I think that postal voting has real problems - as I've been discussing over here (worth reading the post that's attached to too, it's very interesting).
The basic problem boils down to this - if you're voting from home then there's nothing to stop someone bribing you to vote X, and then sitting behind you while you click the button/write on the form. Similarly, much more social pressure to conform can be exerted. Whereas, if you're voting in a booth then you can say "Yes, I voted X." when actually you voted "Y", no matter how much social pressure is exerted. See The Times article on that link for an example of that kind of pressure being exerted.