@Marsu42:
Actually, the only thing I've shot with the 5D Mark III was the timelapse above, so I can't really give a good advice yet.
For now, I can say that LiveView is actually a lot more usable in darkness on 5D3, compared to 5D2, probably a 2-3 stop improvement. Certainly a lot more than the 0.5 stops rumoured on some forums.
Also, if you shoot in burst mode, or if you care about battery life, go for 5D3. When shooting the timelapse linked above, I've measured 6 hours of battery life during 15-second exposures with intervalometer set to 20 seconds. That's impressive IMO.