My one issue with bladders is mold growth. Even when empty, they remain wet inside, and wind up growing colonies of the bad stuff unless there is some way to completely open the bladder and air out every square inch of it.
I am a fan of Waterbricks. They cost $35.48 + shipping for two, are easy to air out and sanitize (toss a tablespoon of bleach, add water, shake, let sit overnight, dump), they interlock and stack up to four feet high, can take a heavy pounding, and are individually not too heavy (at 3.5 gallons, weigh about 30 pounds each when full), seal well, and be stored open, so they completely dry out and not invite mold/mildew growth. You can get half bricks, which hold 1.6 gallons.
Of course, they can be a tempting target for thieves, but a tonneau cover should deter opportunists.
