Kudos for starting a purchase with battery considerations! :twothumbs
No, AA and RCR123 / CR123 are not interchangeable, neither in charger nor in the flashlight. And that is not only because of the size. . .
The RCR123 have a higher voltage than AA because the chemicals are totally different. There are several chemistries for the RCR123, but they all involve lithium and are rated at 3.7 volts or thereabouts. Today, the most used rechargeable AA batteries are based on nikkel-metal hybrid (nimh) and are rated at 1.2 volts.
Since the voltage is different the rated mAh capacity shouldn't be compared directly. To compare you would have to calculate the watthours, which is the correct measure of the amount of electric energy in your battery (such as 900mAh*3.7v = 3.3watt hours; 2700mAh*1.2v = 3.2 watt hours - not really that different). But also other characteristics are important - such as the ability to deliver a high current without sagging too much in voltage, and self discharge (I.e: How long can the light sit unused before the batteries are dead?). If you want the brightest possible light in the smallest possible package then lithium based batteries is the way to go.
In everyday life (real life?), other factors are just as important. AA are found everywhere, RC123 are found either at a high price locally or you buy them online. Rechargeable lithiums (RCR123) have some security issues you should be aware of. (This shouldn't be a problem if you use high quality protected cells and a good charger and pay a little attention to what you are doing). Use the search function to find more info about this.
Either way, I'm sure you will be satisfied with both L2D and P3D. Personally, I'd stick with the AA format and get myself some low self discharge nimh batteries.