More the same times 2, but one is way better than the other.
Extraction Point starts just where the original game ended. Extraction helicopter crash lands in the vicinity of Origin Facility and now it's up to you again as the Point Man to survive to a new extraction point. On the way you'll learn more about what is really going on, but unfortunately the F.E.A.R. saga doesn't reach its conclusion in the end like it did in Doom 3's Expansion Pack Resurrection of Evil for example. Instead you are left wanting for more or being sorely disappointed by the story.
Extraction Point is a clear case on how a game can be made better: it's superior to the original game in almost every way. Graphics have much more color, environments are more varied, interactive and most of all: much brighter than last time. Open spaces are used more instead of just narrow corridors one after another and the story is told through cutscenes instead of phone messages and laptop computers, which you need to find yourself. Point Man is in contact with other F.E.A.R. members a lot more this time around, which creates a feel of team work in the game. Also checkpoints are much more frequent and the game is not too long: it ends just when it starts to feel boring. Good thing is also that the focus is now more on exploring and creating a scary mood instead of just straight run-and-gun shooting.
Audiovisual presentation is almost identical to the original game, but now music is used much more and it's a great component in adding a feel of scaryness. Other thing is that the sound of firing a gun is now a lot more satisfying.
In the end Extraction Point is a great addition to the saga of F.E.A.R. and it brings hope that maybe the upcoming F.E.A.R. 2: Project Origin can actually be an A quality FPS game.
Perseus Mandate is another story, because it happens side by side the original game. You are a member of another F.E.A.R. team named just as Sergeant. For some reason you also have that same super reflex thing (slow-mo) the enhanced Point Man had. Since this game happens at the same as the original, its only function is to clarify things that happened in Origin Facility. Story moves forward via in-game cutscenes and radio dialog instead of voice messages and laptop computers. This is a good thing.
You are now even more part of a team fighting alongside them much more frequently than ever before. Unfortunately this also means that exploring and mood has been changed to a straight forward run-and-gun action. In short it's just boring.
Graphics are the worst in series' history: character models are blocky and there are no details in environments. Navigating through these levels in Perseus Mandate doesn't give you anything new in pretty much any way and on top of that the frame rate takes a serious hit way too often. Luckily the audio side is done as well as it was in Extraction Point.
Perseus Mandate is an unnecessary add-on to the F.E.A.R. saga so my advice is to skip it entirely.
The main question still remains: should you buy this game? If you enjoyed the original game then by all means check your bargain bin. Just be prepared that only one of these expansion packs is worth playing.