If I were going to buy into the modular Sennheiser series and purchase a MKE-2 lav and ME64 or ME66 to use as a shotgun, which power module is best? The K6 that works off battery or Phantom or the K6P that is 'Phantom only'.

When would you use the battery only configuration of the K6 mated to one of the Sennheiser mics?

I don't know why one would want to buy the P version since the other version will work with either the battery or Phantom unless size and weight are really a problem.

If you don't have a phantom power supply available, like with some of the plug-on wireless transmitters or with most XLR adapters, the battery option is very nice.

Another issue is price. The "P" model is higher priced simply because it's not stocked a lot and is produced in shorter production runs than the standard preamp.

The "P" and standard K6 are identical in every respect except the phantom only option.

Yeah, if you're only getting one it makes no sense to get the K6P. You should know however, that a lot of people claim the K6P sounds better. We have both, and I can't hear the difference. Personally, I like the smaller size of the K6P a lot better. In fact, it's all we ever use unless the mic is mounted on-camera.