At the consumer end of the market there isn't much in it. People tend to buy into one lens system and then stick with it due to sunk costs. There's also just enough difference in user interface to make it easier to stick with what you know, and the time invested developing an understanding of it.
Even at the semi- and pro end of the market they tend to leap-frog each other, and any current reputation (e.g. Canon have been strong in HD video, Nikon took the lead amongst portrait strobists) is subject to overnight change at the next major product announcement. There seems to be frequent changes from one to the other by big name blogger pro's, no doubt assisted by the relevant marketing departments at one level or another - but showing that the brand on the box is less relevant than the quality spec of the gear.