It used to be that alkalines would outlast rechargeables (in the NiCd days), but technology has improved and thus the capacity of our AA sized secondary cells is now higher, while their internal resistance is lower - therefore they can deliver more current without too much 'voltage-sag.' I would imagine that lithium chemistry could improve a bit and we'll see lithium secondaries outlasting the primaries by a long time. That said, I have no experience with lithium primaries at 3.0v vs lithium secondaries. I'd never ever use a primary cell where a secondary one provides better performance price-wise. Surefire has their act together when they refuse to support secondary cells, they want to sell you their batteries at $3.50 an hour to run your light.. good idea, for them!
You'd have to calculate watt-hours. Remember that Li-ions are 3.7v, multiply that by say 600 mah for a typical RCR123 and you get 2.22 watt-hours. (the attraction is the form-factor, it takes 5 AA nimh's to get to 6v, and 2 CR123 primaries!)
For calculations let's go with a pretty serious 2000mah Nimh, at 1.2v, and you'll get 2.4 watt hours, slightly more than your lithium RCR123, but at a much lower voltage you'll have to draw more current, or have more cells.
My personal favorite which I just got my first light for (and LOVE the form-factor), is the 18650, a completely different size class, but at 2200mah for a good 18650, and 3.7v working voltage, you get 8.14 watt-hours :-D.
