We really like the 6P because of all the upgrade options for it, both from Surefire and aftermarket. From Surefire in recent years there is the KX4 and P60L, but from the aftermarket there ranges all sorts of options from a very inexpensive DX module all the way to the Malkoffs and the Moddoo/Oveready Triple-XPG.
The E2D is no slouch for upgradeability either, taking the KX2 from Surefire, several hotwires from LumensFactory, and the
Kuku427 LED tower module. But the 6P is still far more upgradeable from a much larger system of parts.
I would lean in the direction of picking up an E2DL first, and then a standard 6P (not LED or X) later. The E2DL makes a great pocket light and the (upgraded) 6P will be a great all-around light to compliment the E2DL.