Upload & Sell: On
Although I've not used the Nikon external powerpack ... I've never been impressed by OEM offering a case with a cord that simply holds more AA batteries. I much prefer something like the Lumedyne Tinycycler, Quantum Compact or other battery options. So for me, the external powerpack option isn't a strong selling point ... which btw, is available for use on the 800 as well as the 910, according to Nikon literature (links below).
After a quick review from KR (yeah, I know) ... the thing that catches my attention is the slightly faster recycle time of the 910. The power differential isn't significant (actually rated less) ... and likely are very close in reality to the same @ limits of flash tube. I truly don't see a POWER gain by going to the 910.
The 800 seems to have 3 channel wireless as well, so no real gain there that I see either. I'm not a Nikonian, so others can better speak to the diff here, but according to what I read @ Nikon ... the 800 is commander enabled likewise.
So, unless I seriously need the (slightly) greater recycle speed for full power flash on AA batteries, I'm going with the 800 and putting the money saved into a REAL BATTERY, like a Quantum, etc. According to Nikon's literature ... the gain between the 800 vs. 910 is .4 sec (@ full power on NI-CD). That diff is going to be even less for flash at less than full power, in real world. I wouldn't pay any attention to the "2 second" difference on slower batteries, because if recycle speed is truly needed ... you won't be using slower batteries anyway.
This is what I've got under my camera.
Other options abound, but I'm just not impressed by Canon or Nikon's external powerpack options to simply buy more AA batteries. Combine that with what seems to be the flash tube being a limiting factor @ power output ... the "latest & greatest" just seem to be an exercise in 90% more money, for 10% more performance (and some re-packaged bells & whistles) in many instances, once you boil it down to the nuts & bolts.
They seem to be touting some minor changes to the zoom head, but both hit 14mm using the diffuser, and if you're putting it in a box ... I don't think the longer zoom is going to be of much value there either.
I'm leaning 800, unless I've overlooked something of significance along the way.