It can also depend on the local shops - a local shop might well push one brand over another (either planned or its just what the shop staff "like" more) and as a result when they also advertise training courses you end up with a heavier population of one group over the other.
As for which is best you've two options to work it out:
1) Flip a coin
2) Have very specific requirements and then put the brand as a whole (lenses, bodies, flashes, etc..) to the test of which meets the needs for those requirements the best for your budget. Note chances are you might well find that it doesn't actually matter or that if it does the margin of gain is so slight as to be a null point.
Honestly there is nothing you can do with a Nikon that you can't do with a Canon and nothing you can do with a Canon that you can't do with a Nikon. Heck if you restrict yourself to own brand lenses only there is only 1 lens that actually divides the two brands and that is the Canon MPE 65mm macro which is a darn rare lens (even in macro circles its the luxury/rare option). And heck you can get better results than it can produce with some alternative setups (although you tend to lose the 1:1 to 5:1 "zoom" feature and the alternatives might well cost you more and require some DIY)