To my eyes, the MN15 is just a little whiter, but the beam is a little more oval. The N2 is almost a perfectly round hot spot. I don't remember the spill, but I imagine they are almost identical. :)
I find that the MN15 lamps I have produce whiter beams for longer compared to the N2 lamps I have. I also find that the MN15 lamps produce more light with a larger hot spot. Granted this larger hotspot is squashed and oval rather than round/sub round but there is more light.
The N2 beam is usually more pleasing but in use the MN15 is the better lamp assembly in my opinion. That said, I use either the N2 or the MN16 :D
I do have the ability to compare many beams side-by-side and although there is some variation from lamp to lamp and from reflector to reflector (from bulb-reflector combination to bulb reflector combination) I consider my findings to be characteristic of all N2 and MN15 beams I've seen.