if you can afford it the mh will be better for veg, you'll see faster growth and tighter nodes, which will help if your not planning on growing trees.
as far as grow style is concerned you cant really go wrong with lst but if you wanna take it a step farther scrog is the way to go for small spaces.
in the 2x4 area a 400 watt light will give you 50watts per sq foot, that should do you just fine. for a 2x2 area the 250 will do the trick and then some. remember watts=lumens=dense heavy buds
either way with the vortex fan you shouldn't have too many problems with ventilation.
have you thought about what your gonna do for intake air?