I have been testing a few of the new Fisher 29ers with the G2 geometry. I have noticed since the switch to the G2 that the bikes handle a lot better then they did in the past. On the right trail one can ride over just about anything and not worry about slowing down. The biggest down side is on steep slopes the 29er are much harder to ride. So all said if your just doing straight XC stuff the 29 is a good choice, any sort of tech stuff stick to the 26".
Nice thing about the Mamba is that it shares the same frame as the other 29er hardtails so you can take all the parts off and build up your own bike.