The XTR stuff will be lighter, supposedly better shifting although I'm not sure how many people can actually tell and will relieve you of more money. If you want brand new, both have moved to 10 speed only. The new Shimano stuff is getting great reviews and appears to have won some people back from Sram although some people do find the new XTR shifter's action too light.
With regards to the hydration pack, I use an Evo C 16c which will take up to a 3l bladder although I'm using a 2l one t the mo which I may change for a bigger one when it gets a bit warmer.
2015 Stanton Ti Switchback
2006 Specialized Demo 8
2013 Specialized Allez Sport
1999 Santa Cruz Chameleon
1994 Cannnondale Beast of the East (rebuild still in progress)