for a good knife edge in a plane you need good power to weight ratio. You'll be flying on the side of the plane in theory so theres a lot less 'wing' area so it'll need more power. Also an effective rudder has to present so it can keep the fuselage in a knife edge flight. Also a deep fuselage is a MUST. The limbo dancer will not knife edge as its very thin in height terms. Look at something like the flip 3D, that'll knife edge fine as it has a lovely deep fuselage.

For a prop hang you again need a good power to weight ration, obviously power has to overcome the weight. The lighter you can get the model the better. If you 'fall' off the prop then you need some more power just to recover from it. Also large control surfaces are helpful as a lot of air has to be shifted to keep it in vertical. I have a VMAR arrow which is a typical low winger aerobatic model. It can also just prop hang but i'll have to be careful as it doesnt have power to recover from it if it falls out of the prop hang. Also i'll have to modify the elevator to give me enough area to keep the plane vertical.

hope this helps