I guess its what they get used to, all of mine up until the last two had bottles, but I lost my two old girls within a fortnight, both of the new girls came from a friend who had bowls for hers, my gorgeous Tinkerbell didnt have the time to teach her companion how to use the bottle as opposed to the bowl. So bowl it was!
Bottles are more convenient & get less messy, but bowls are easier to stop freezing in the cold weather - with bottles the spout always needs checking its not frozen solid. Also in any weather if you under fill them the bottles tend to drip.
in warmer weather Bowls can be used as paddling pools - probably not very hygenic- but was great fun for my piggies! @)