Do you mean The XP-G R5 versus the XR-E R2? Ot the XP-E R2 vs. the XP-G R5?
R2 and R5 are flux bins and are not the best way to identify an emitter. They're only useful in comparing the same emitter.
Let's assume you meam XP-G's all 'round.
What will affect your runtime is the current that the emitter is run at. A R5 and a R2 will both run just as long as the other, if they're driven at the same current. The R5, however, would be brighter.
If you were striving for exactly the same brightness, the you would run the R5 at a lower current than the R2, and you would have increased runtime.