For the 6P it comes down to what's more important: brightness or runtime, and whether you require/want incandescent or LED.
For brightness with incandescent you should look at the Lumens Factory HO-9 on 2x 3.6v RCR123s.
If you want to go LED, there are a great many Cree drop ins that will give you great output and decent run times.
For the 6P I favour the Crees these days, I've got an older drop in (can't even remember the bin - it was a pretty cheap unit from DX and is still great) and the modern Q5s would appear to be even better. The Malkof drop in (if you can get one) appears to lead the pack, given the opinions expressed by those lucky enough to get their hands on one.
I'd have to say, unless you really want an incan, the cree is probably the way to go for run time and brightness in the 6P siezed package.