Happily though you are not forced to stick to one style and I don't think that you miss out on any story elements by swapping between styles. This is important to me partly because I like variety but also because I tend to engage in a bit of subtle role playing in a game like this. I won't choose a course of action just to get the next achievement but instead I like to act in manner fitting with the protagonists mind set. When highly trained special agent Sam Fisher encounters a humble security guard just going about his job it makes sense that he would try to sneak past unobserved or at worst put the guard to sleep for a while. On the other hand when he encounters room full of ruthless killers who have just shot up his buddies there is no way he would let them wake up the next morning with nothing more than minor headaches. He is going to shoot as many real bullets into them as he can.
I find that this almost subconscious role playing greatly enhances immersion and my enjoyment of the game. I remember doing it in Deus Ex HR where my character gradually morphed from non violence into furious vengeance as he became more and more aware of how nasty his opponents were.