I find this interesting because my initial reaction to placing stuff in a rucksack was one of weight distribution.
Stuff on your bike is lower and so will the centre of gravity, you + stuff + bike will weight the same so lower should be better.
I also realise it probably makes no difference either way, but it's good to get the grey cells working.
I think any disadvantage of having it on you rather than the bike is offset because you use your body (and anything attached to it) to move the CoG around as you ride, moving it sideways while cornering, front/back while climbing/descending. On a hardtail you could also regard anything on the bike frame as, effectively, unsprung weight.
That does seem a bargain.
Does the bladder on any of these interfere with internal space at all?, ie does stuff rattle around as you drink the level down.
In my camelbak (and I assume most/all makes are the same) the bladder is in it's own compartment with padded/insulating material between the bladder compartment and the first storage compartment. As well as protecting the bladder from tools etc.. it also stops your "junk" rattling around.
The bladder compartment also has an additional external zip that "shrinks" the compartment to make the whole pack smaller if you're not carrying more than a couple of litres.
The bladder on mine takes around 5 litres in theory but in practice I rarely have more than 1-2 litres in there - if you still have more than a mouthful left after a ride you took too much.
Mine's a fairly small pack but there's enough room to carry waterproofs, tyre levers, fork pump, snacks, phone, spare inner tube, puncture repair kit, multi-tool and any other random crap that I throw in there at the start "just in case" and then wonder why the pack weighs more than I do