if it fits it's fine, if it doesn't it's a waste of time so try it on before you buy.
at the EOTD it all depends on what you want - minimalist approach like this one, or one with a bit more carrying capacity for spare kit, keys, phone etc. alternatively, many prefer belt bladder systems like the C'bak Flashflo for running with as then there are no shoulder straps to rub (is what I use). but not everyone is happy with belt systems.
C'baks tend to be at the premium end of the market along with the likes of similar offering from Platypus, Source, Deuter, Salomon etc. you can cheap lookalike versions at times from places like Tesco, Aldi, Lidl etc.
as you say - so many to choose from!! but the key always is correct fit.