Oh boy, here I go again with another of my overly informative, entirely useless textiles post: (Hey, maybe I should do this for a living instead of going to university!)
Nikwax is a silicon based product. It's environmentally friendly or "green" if you will. This is why it's so popular.
Grangers, Storm and the likes are flourocarbon based. These are NOT environmentally friendly, so much so in fact that in my city, our major outdoor gear stores refuse to stock it anymore. I was lucky enough to nab one of the last bottles of Storm proofing sometime ago, and I'll be damned if I'll ever find it again.
Personally, I much prefer flourocarbon based waterproofings. In my experience, and I've tried Nikwax on numerous occasions, Nikwax just doesn't cut it. The proofings don't last nearly as long as Storm or Grangers.
Anyway, it's really up to you the consumer to decide but I'm not much of a Nikwax guy anymore.
Cheers and I hope this helps!
Adam