I think this is a mess in our language (choosing what form of imperative sounds the most pleasant to the ears).
In the TV ad. from Caixa Economica, they say: "Vem pra Caixa você também", and this is more pleasant them "Venha", which, in my point of view, sounds more urgent. But vem is the 2nd person imperative, while venha is the 3rd.
Anyway, we cannot control how it evolves. :)