do yall prefer the all-LED A2L or the incan/LED combo A2 if you had to choose? and why?
Absolutely no question about it, the incandescent A2.
Why? It's the only commercially available regulated incandescent light, and to my knowledge the only pocket-sized regulated incandescent light. You can regulate an M6, or a Maglite, but they're not pocket lights!
The combination of low-current LEDs for navigation, and a regulated incandescent bulb with more throw than its size would make you guess, is just excellent, even now in 2012. The A2L is a mediocre LED light with a mediocre LED. It doesn't have the 'it' factor of the A2, nor any special features beyond what others do, besides having some different colors.
Are the heads found on the 4 flats the same as found in the round body A2 Incan?
As far as I know the internals are the same. However, some of the old 4-flats have completely flat bezels, whereas newer 4-flats and the newest roundbodies have slight crenelations.
Disclaimer: I've never actually seen a 4-flats A2 in the flesh!