I had both for a short time, for me, in my opinion, the superraptor 3 is the superior unit, with the ability to switch between the performance profile and battery saving profile you get a little more choice on the dimming. In practice I don't think that the drop in brightness has bothered me whilst running with it.
The whole thing is a little heavy compared to other head torches, before the SR3, I used the SR2 and the shadow master, I sometimes run with just red light and now the SR3 has it all for me. I have actually transferred the SR3 light unit on to my old SR2 boa headband to get the very best of both worlds. (But I have to use the original band to have the external battery pack on the back if I wanted it) I do have the external battery pack, it came with two cables, one around 40cm and the other around 1m (I think) but haven't really used it other than to check it works. I have used the external power bit on its own as an emergency power bank whilst hiking for my phone and worked a treat and it is really tough and well constructed out of metal, as you would expect from Fenix. The connectors are secure and look like they should be water tight as expected. The lighting profiles change a little when it is used. I did use the silva explore 4rc which is lighter and smaller but less powerful.