Well, the performance is better. AA cells are nominally 1.5 volts vs. the 3 volts for a CR123A battery. A good alkaline AA is around 2700 mAh vs. around 1550 mAh for a single CR123A. So in the same size light you can generally have one x AA or 2 x CR123A. You definitely get more lumens from the higher voltage lights as a rule. The re-chargeable cells are even more powerful but lack the long shelf life/low-self-discharge rate of lithiums and aren't as good in the cold.

But to me switching to lithium batteries is what completely changed the equation for me. The best price I can find on lithium AAs is generally around $1.75 each (Energizer Advance Lithium- the Ultimates are even higher). I get CR123A batteries for $1 online. My two most used lights both run on 1 x CR123A.

To me CR123A makes the most sense. I have at least 8 flashlights that use them, plus one weapon light. I still have a few lights laying around that use AA or AAA, mostly very small lights like my Fenix EO1. But for the bulk of my uses CR123A suits me best.