If you get the twinflash (as you are) I'd also suggest getting yourself a Pixel Battery Pack for the flash (takes the same attachment as the 580EX2). I say that because the twinflash is older, so it recharges somewhat slower than more modern flashes and also when it does charge it lets out an annoying audible whining sound.
If you get a battery pack it will help charge the flash faster and also cuts down on the whining sound. Granted you can't fire it like crazy, but I find it much more usable with the battery pack. Plus with two sets of 4AA batteries charging it you can go a pretty long while before you've got to change any batteries around.
There are other battery packs on the market and Canon also makes one, the Pixel I think are well priced and well featured whilst not being bank breaking not too big/heavy. Certainly some of the more heavy duty battery packs might do a better job, but they are going to be bigger, heavier and more expensive (and likely overkill on the flash).
Tools of the trade: Canon 400D, Canon 7D, Canon 70-200mm f2.8 IS L M2, Sigma 120-300mm f2.8 OS, Canon MPE 65mm f2.8 macro, Sigma 150mm f2.8 macro, Tamron 24-70mm f2.4, Sigma 70mm f2.8 macro, Sigma 8-16mm f4.5-5.6, Raynox DCR 250, loads of teleconverters and a flashy thingy too