How to choose the right hydration pack for me
Rule of Thumb: a water bottle an hour

That rule of thumb slides from person to person... me I am more like a 1/2 bottle per hour a little more if it is hot and a little less if I am mountain biking with my dog just joying being outside.  

On a 3-4 hour road ride with no water stops available along the way I would take two bottles... if it was exceptionally hot I may add a small bottle to my jersey pocket.  On a similar time duration mountain bike ride (sans dog) I would use a hydration pack.

So, things to consider:


Do you just want the pack for hydration or do you want to carry stuff in the pack as well (e.g. Tools, pump, jacket, food, etc..)
After gauging your consumption rate (only you can do this) and considering  a bottle is 22-26 oz determine the bladder size you need/feel confident will provide the amount of hydration required for your targeted activities.


Once you answer those questions I am pretty sure you will be able to look at your selections and make all the little personal preferences relative to style and selection necessary to wind up with a hydration pack that works for your needs.