The Hauser pack is the only hydration pack in the entire world that makes you look cooler than not wearing one. There are pockets everywhere, it's super comfy to wear even for full days out in the middle of nowhere, and none of your mates have one because they've traditionally been rarer than a chook's chompers.
The Acre Supply (by Mission Workshop) 10L Hauser will fit a day's worth of MTB stuff and a 3L bladder and comes with enough straps on the outside to carry your helmet and shoes off-trail, and the 14L will fit all that and an extra... like... four litres of stuff.
** Please note: 14L Camo Hauser does not come with a Tool Roll, that's why it's the same price as the (normally cheaper) 10L Blue one. Please don't expect there to be one in there, you'll waste valuable trail hours searching for it. **