The only difference is that the 910 is newer. They put out a TON of juice and after a recent workshop, I've begun a serious love affair with these bad boys. I can rock some super fun lighting setups, which I love because it gives me more creative control in an outdoor or indoor environment where it's quite possible (or kinda guaranteed) that the existing lighting may need a bit of help.
I also love how small & portable these lights are compared to my studio strobes. Now, granted, my preference is to shoot in my studio but I have been incorporating these little powerhouses into a few more of my studio shoots even when I'm using my strobes as primary light sources. The most important thing I can say about speedlights is to get them OFF your camera. On-board flash is never flattering but taking it to either camera right or left and placing it at a 45 degree angle to the subject, NOW we're making some magic! I also use a Lastolite portable softbox with these flashes. It replicates the soft light effect that I love in the studio and I gotta say, being able to bring these effects to location shoots is preety fabulous!
I also purchased Rogue Gels for my flashes. They are extremely cool and fun to play with. I have not spent a huge amount of time getting a feel for all the incredible effects achieved by these gels but just knowing that I can alter the color of the flashes makes me all creative and jiggity!!
Have you ever played with modifying the color of the light you're shooting? If so, how did you change the light color/temperature and what did you think of the results?