what is the difference between the two?

 

Self aligning does not require guideplates, whereas non-self aligning does require them (this is what I have heard anyway)

 

The difference is in the method of keeping them aligned on the top of the valve stem.

NSA use a guideplate that keeps the pushrod from deflecting from side to side. This keeps the rocker arm centered on the valve. Hardened pushrods are required with the guideplates.

SA uses little guides on the tip of the rocker at the valve to keep it centered.

NSA are probably considered the more stable of the two especially at high rpms.