I often debate whether I should be using handheld bottles or a large capacity reservoir for my fluids on an ultra. I have been running with the Ultimate Direction (UD) family of running vests for some time, alternating between the smaller AK race vest and the larger PB race vest. I find that I like carrying my fluids high on the front of the vest as long as the vest can be cinched down to prevent bounce. High on the back is OK as well if there is room for a reservoir. I use the left front water bottle pocket for my phone and the right front pocket for a 20 oz bottle. A 24 oz bottle, by the way, becomes excessively top heavy and bouncy when full and riding in a front UD vest pocket.
But the diminished fluid carrying capacity of