Personally, I have an Energizer Energi To Go 18000mAh external battery. The value of this is that I can charge my laptop, n900, pocket Wi-Fi hotspot, iPod & DS3 controller all with the one piece of equipment. That said though, I have a pretty decent-sized backpack to cart it all around in.

There's pros and cons to either argument, but if you're at all concerned about portability, I'd probably go with the Mugen option. It means that if need be, you can just slip it in your pocket and head out. Plus it has the added bonus of solving the issues people have had with the blue line on the shutter & picture quality. You may want to read some reviews of it, but if I remember correctly due to the added bulk it's actually more than capable of standing up without a kickstand.

EDIT: Guess I'm wrong about the last part!