Well, I am not here to stop the stream of Borderland recommendations, but I say that it all depends on what you are looking for.
I have had every single iteration of NG except NGS2. I must say however that Sigma was an excellent edition of the original. On top of that, NG2 was an extremely awesome game, and from what I have seen of Sigma 2, it is only better. That however is a badass single player experience for the most part in which the focus is on melee combat and extreme battles.
Borderlands is however an amazing game as well. Like others have said the main thing is to play this with friends. So unless you like playing with random people on the PSN or have multiple friends who will have the game, I instantly say get Sigma 2. However if you do fall in one of those earlier categories, i would say it all comes down to which type you are looking for. Borderlands is an excellent multiplayer adventure whereas Sigma 2 is mainly single player.
If I had to pick one, I personally would go with Borderlands because I have enough Ninja Gaiden in my collection right now. Something to note, that all of the next gen versions of Ninja Gaiden have all dropped in price quite drastically after release.
I hope that helps a little at least!