+1 with Nathan (mine is a HPL), have done a couple of marathons with mine, really comfortable with almost no bounce. Large enough for a 2L bladder, few snacks, water-proof top, water bottle, phone, emergency blanket.
Also have a Raidlight (Olmo 5), which I've not had the chance to put too many miles on yet. Large enough for a 2L bladder, lots of snacks, water-proof top, 2 water bottles and phone, emergency blanket.
Big thing I found if using a bladder is to remove as much air as possible to stop any sloshing.