Does your story begin in the midst of a zombie apocalypse? As the type of 'zombie' should be based around the way in which the zombies have come about. If there is no back story for their arrival then you can go for any type you want. Also, it depends what kind of feel you are going for, mindless zombie killing fun and action (Dawn of the Dead, Zombieland etc) or a post apocalyptic, dystopian feel (28 Days Later). If you want the audience to feel threatened and scared by this 'invasion' then definitely go for the fast, infected type as it has a sense of reality to it, the idea of infection is easily believable and some do believe that something like that is actually possible and will therefore create a tenser more atmospheric film. If going go for just an action, slasher genre then take the slow zombies, everyone loves a bit of zombie brain smashing. But as I say, it depends entirely on what audience you are aiming at and the effect you are trying to create