Yes that's true, his ability does have a visual effect to it, allowing people to see if. But as Benjamin showed its an illusion, wind cannot stop it. So hiding behind something like a wall may not even slow it down. It just keeps following you without stopping or being hindered by anything (Except Bella's freaky brain).
Alec to me as always seemed the kinder, but probably not by much. Like with most vampires, he can be a nice guy, but hit the right switch and he is gonna show you how dark he can go. I always saw him manifesting his power as a way to lessen the pain of his sister. Imagine the one person you have in life, the only one who means anything to you screaming in agony. Who wouldn't want to take their pain away.
Alec to me has the more go with the flow mentality, his sister is with the volturi so he stays with them. I cannot imagine Alec without Jane. They are just to close, they are each others support the ones who know everything about each other (Save for Aro). If Jane died, I think it would be like Alec lost his mate, while they are (At least I hope) not intimate they are each others other half, they are so closely intertwined that you cannot really see where one ends and the other begins most of the time.
Alec's power is by far more psychologically terrifying, knowing its coming for you, knowing that he can make sure you never escape it, and knowing what it does.. that's true terror. I see him using his power on his human prey, among all the screams and terror of the 'Dinning Hall' he could simply touch them and end everything for them in one second (By touch I mean get so close then his fog would pretty much instantly effect them) and then feed without them feeling any pain. Passing into darkness without feeling a thing sounds better then dieing in pain, most of the time.
If Alec was inclined I think that he could destroy the Volturi, and simply do it by leaving. I have a feeling that the guard's fear Jane because of her use of her power, I think they are terrified of Alec because of how powerful he really is.. you cannot get close to him without losing your senses and once that is done.. as Kate said 'All it takes is a second'.
R.I.P - Jacob Black [Died August 2nd, 2008](Page 360, Book 2, Breaking Dawn)