As I understood, -ve plane is transparent, it needs les memory space, so while opening a file it takes only less time to load the file, compared to +ve planes.
I prefer -ve plane because of this.
But, I too have seen boards with +ve planes, (I was so uncomfortable working with that) designed by experts (I think). I did n't understand the reason; may be some fabrication people prefers +ve planes.
expecting for further discussions ....let us share our knowledge.
thanks & regards