Kevin, there's actually two different styles.
On the left is the SW-02, and on the right, the Z48. It's my understanding that the Z-49 is the black Type II anodized version of the Type III hard-anodized Z-48.
I have both (SW-02 annd Z-48) of them and have had the ability to putz around with them for a while on both my M2 and M3.
The SW-02 is a kick-ass switch, and the extra width helps keep the light very firmly in your hand (preventing forward creep) while using the switch, especially on the M3, with its elongated body. It looks great on the M3, but on the M2, it looks like it gives the light too much "ass" because of the change in proportion. The larger bezel of the M3 balances very well visually with the SW-02 on it. In addition, the extra bulk at the tail end is less important on a larger light that's much less likely to find itself stuffed into a pocket.
M3 with the SW-02 switch tailcap.
(Photo from Sean at SFDB.)
M3 with Z-48 tailcap and bezel filter assembly.
(Photo from PK Engineering website.)
The Z-48 gives both the M2 and M3 their slender looks back. While it works great on both lights, I've found that I prefer it more on the M2, it doesn't overpower the light dimensionally or visually. The shorter body of the M2 also allows sufficient grip without the added diameter of the SW-02. It also give less bulk to the M2, important with a light that's sized to fit in a pocket.
M2 with Z-48 tailcap.
(Photo from PK Engineering website.)
My only negative comment would be that the rubber crown is a bit tall for me, being cursed with wide palms but short fingers, so it's sometimes a bit of a "stretch" to hit the button properly. I'll probably end up trimming it down a bit.
Dan, your 6P is the original Type II black anodized, IIRC, so the Z-49, on the left, is your ticket to happy clicking.
(Photo from PK Engineering website.)