The XP-E Q5 is less efficient by about 20 lumens/watt but has a tighter beam (might throw a little better). The XP-G R3 has a floodier beam but is more efficient and a better l.e.d. overall. Both l.e.d.s have been around for at least a couple years. Look for something with an XP-E2, XP-G2, XM-L, or XB-D for a brighter more efficient light. These newer l.e.d.s have dropped in price compared to older l.e.d.s so you might get a better deal if you can find the light bulb you want with the newer l.e.d.s. The l.e.d. in the light is not the only determining factor for cost. One might use several l.e.d.s while the other may only use one. They may be driven at different currents causing one to be brighter than the other. One may be better built with a better thermal path (heatsink) making it last longer. Branding could also play a factor. It might help if you told us the specs of the different bulbs so we didn't have to guess so much.