'Get away' from me...'You better back off before I smack you one.' means that they are a little toooo close to your body right now...
'Stay away' from me...'Do not come near me or I will smack you one.'...means that you do not wish to see them again..
Usually, "get away" means that the person must move away from someone or something -- he or she is still too close -- , and "stay away" means that the person must not come close to someone or something -- he or she is already far enough.
I generally agree.
I say "get away" if I want to be left alone or don't want someone to come near me. I use "stay away" to re-inforce it. I may also use it if I want to stress more that I don't want whomever to get anywhere near me.
I suppose it is a matter of preference too.
"Get away"--means you are already too close.
You say "stay away" when you do not want them to get near you in the first place.
You can say, "Stay away from me!" on the phone or in a letter. I suppose you could say, "Get away from me," also, but "get away" has the distinction in that the person saying it already feels that the person (either by phone, e-mail--by whatever means) is already too close.