I have one of the new Quark AA2 XPG neutral whites. To my eyes, the neutral white looks better; it seems more contrasty on what I shine it on so it gives better depth perception. I don't notice any difference in brightness between it and my Quark AA2 R2, although the R2 has a tighter more defined hot spot and throws better.
One caveat is that my neutral one definitely has a greenish cast to it. I can see it when white wall hunting, but it's not nearly as noticeable outdoors during actual use. Still, it just looks odd to me... perhaps it's the fact I know it has that greenish cast to it, but I can't help but get this feeling that things don't look quite right under its beam. It is still an excellent light though, I just prefer warmer neutrals, like the Q35A, which is easily my favorite tint, so I prefer my Jet-III Pro over the Quark.
I am looking around for a good headlamp now, and planning on a Zebralight, they have Q45B's in their neutral whites, so I'm looking forward to seeing how that one compares.
The moral of the story is that while the Quarks are good lights, a good value for the money, with the current neutral batch you've got a big IF in the tint lottery... so if you know what tint you want, then you might want to consider another brand that uses a specific emitter that will give you a tint closer to what you want to have.
