Metal Gear Solid Ground Zeroes is kind of like a prologue to the actual fifth installment of Metal Gear Solid, however, despite this, it is still being treated like a full blown game, and unless you buy the pack with access to both titles in, it will cost just as much as a full game too.
Unfortunately though, I don't think Ground Zeroes will be anywhere as good as the Phantom Pain is looking to be, and one of the reasons for this may be because the story mode on Ground Zeroes may end up being ridiculously short.
And by ridiculously short, I mean less than two hours short. Of course, it's hard to know whether this is the true average play time, or the time it would take to complete the game when you already know everything you need to do, and completely disregard all of the other cool things to explore in the world.
Some are saying that the game plays for about 2-5 hours on average, but according to Hideo Kojima's personal assistant, Ayoka, all of this information is false. The following is a reply on twitter from Ayako.
Despite Ayako's word on it, we'll have to wait for the release of Ground Zeroes on March 18th to truly find out how long the game is. I'd suggest that unless you're a huge Metal Gear Solid fan, don't buy Ground Zeroes until we've seen some proper gameplay footage released after it has been made available to the public.