Personally if I had one set of either I would use that, but if you don't have any to start with I would get Siemens purely because they are still being made, I certainly wouldn't buy siemens if I had greys to start with, there's nothing wrong with either injector, just some tuners won't map greys, sorry can't map greys, sorry that should read can't map anthing.
My overfuelly greys as some call them, that can't be mapped clean low down emission wise idles the car at 0.97% CO, 137ppm, with 180,000+ miles on the bottom end, my L6 couldn't touch it regardless of whose chip was in it.
tabetha