When I mountain bike I have always used a backpack of some sort. The one I own now and have used for about 1 1/2 years is a Camelback Mule. It's great, holds 100 oz. and plenty of room for a tube, co2 inflator, some snacks and a multi tool, with room left over if needed. I currently use water bottles on the road bike, but I am planning on getting a bladder only type of backpack for the coming summer. I have a under seat bag for the necessities, so all I want is water on my back. Camelback makes several nice models to choose from. I would have to suggest a Camelback for a few reasons.
1. Mine has been envolved in many crashes and incidents with trees with no signs of wear.
2. I have had some pretty heavy loads in it with no signs of seams tearing.
3. If it gets real dirty, empty it and throw it in the washer and hang to dry...virtually new again.
4. I know of many people that use them and I haven't heard any complaints yet.

Well that's my opinion, for what its worth
Ride on and drink up