So I was deciding between Ninja Gaiden 2 and Metal Gear Solid 4...

[NJ5]

KZ2 said:

 

(...)

Ninja Gaiden Sigma is apparently a better game than NG2.

Sigma is just a port/remake of a game the OP has already played.

@thekitchensink: NG2 is awesome, but I doubt you can finish it within a few days unless you play a lot!