The Streamlight Polytac LED is better value since it cames standard with a brighter LED and a clicky switch.
From my experience, I would says both the Polytac and the SureFire G2 are equally reliable. They can take a beating all day and still work.
The SureFire G2 does have a major flexibility advantage of the "lego" system. There are a vast amount of upgrades ( bezels, dropins, switches) available for customizing the SureFire. With the Polytac, you are "stuck" with what you got.
Will Malkoff drop ins fit in a Polytac?
The Polytac LED head seems to be one piece. The Polytac body won't take the SureFire Z44 bezel head so P60 dropins won't work.
The Polytac clicky switch does work on my G2 (there is a huge gap) but not vice versa.
Photo of Streamlight Polytac LED, SureFire G2, SureFire 6P :