Mule.

I assume you mean the Camelback variety, not a stubborn grey thing with legs... 

chickaboo wrote (see)

I assume you mean Camelbak, smart arse?

Buy the most expensive (say its a gift and might be swapped), and then give him the receipt as well...

To cover all the bases, get him an Octane 8+. This pack can be adjusted from the size of a mule pack to the size of a blowfish and it has handy energy bar/ tool pockets right on your hip that are easy to access with out removing the pack from your back.

And it doesn't cost much more than a mule.

I use a Blowfish, and I am pleased with it. Have never used a Mule, but Blowfish is lighter than the Mule, carries the same amount of water and can expand to carry more stuff too if you want to. I get the impression it covers slightly more of your back than the Mule though, so it might be hotter to wear. It can be found considerably cheaper than the Mule however (~£40).

Edit: I should add that the Mule is probably perceived as being more desirable, and is an MTB 'standard' piece of kit. Get the Mule, and he is pretty much guaranteed to be happy. I just wanted to share my love of the BF.

I've got both and the MULE is better for longer day's and shitty conditions coz it will hold more stuff.

Funnily enough I thought that the MULE was wider than the Blowfish. When you expand the Blowfish it get's alot deeper.