The reasons why I would buy a G2 (with LOTC) over the 6P:
G2 weighs less.
G2 is more resiliant to drops (better protects the bulb).
G2 feels better to grip, especially in the cold/wet/gloved.
G2 is better at 'hiding' signes of use (dents/scuffs/scratches etc).
Pyrex window is not a must have in my experience of using this type/size of flashlight. I would rather have a Lexan window that won't shatter or crack. Heat (melting/deforming etc) the Lexan is not an issue in my experience. Perhaps when using the P61 for extended periods in hot still air conditions but that is not typical flashlight-using environment for me.
For me, I would much rather get an E2e or an M3 rather than a G2 or 6P. The E2e is a significantly more useful flashlight in my experience (because it is one that can be so easily carried clipped inside your pocket). The M3 is a significantly more powerful flashlight. If I'm going to carry an incandescent flashlight without a pocketclip I may as well carry the brightest!
As for whether the 6P has more compatability than the G2. In reality, they share compatability. There are some body extensions that may not interface correctly with the G2 but the cool stuff like bezels and TailCaps are all interchangeable.