Pi_is_blue,
If you are using NiCd or NiMh, I reccomend using a 4D Maglite. If you are using rechargable alkalines (like Ray-O-Vac makes) you can use either 3D or 4D.
The Trilight is more than three times as bright as the 2AA adapter because the LEDs are run at full power with the best heatsink on the market.
Our main web page has a picture that offers some comparison between the 2AA and the TRILIGHT, but those are over six months old. Our product has improved with time.
The NX-05 optics are held in place by the integral lens of the emitter and the flashlight lens. Any replacement collimnating optics should have a cavity designed to be compatible with the Luxeon emitter. I would not expect a replacement lens without a cavity to be successful.
We sell more 2AA adapters than TRILIGHTs, but as engineers we really like the TRILIGHT a lot more.