The 6P LED is a pretty boring flashlight. It is good because it is quality, easily upgradeable (with both 1st and 3rd party accessories), but it is heavy, and in stock form, not as versatile as say, the Surefire LX2, which I would recommend instead, as it is both thinner and offers a 2-stage switch. The LX2 is not so easily upgradeable (although there are those who offer their modding services to upgrade them), and even better quality.
As to the Quark, plenty has been written recently about them. They are very good, relatively inexpensive, and well-designed. The 123^2 is my favorite form factor for the Quarks.
I find it difficult to actually choose in situations like this. Best to just get both...
IMHO the real decision is how much you're willing to spend and if you want to play Surefire lego or have a light that is extremely versatile.
If you want to play Surefire lego, the 6P is the way to go as there are tons of first-party and aftermarket parts, but if you want something a little more versatile in one package, the LX2 or Quark are excellent choices. If you want something a little more versatile than the 6P and don't want to give up lego, the E2L or E2D are excellent choices as there are plenty of aftermarket parts for these too... although not as much.
FWIW, the Quarks have swappable components too, but they haven't been out as long and are unlikely to achieve the singular popularity that allows 3rd party manufacturers to sell cool compatible parts.