I don't know if anyone cares, but there was a big thing about BPA (a hormone disrupter particularly bad for babies and kids) in plastic bottles a while ago. As far as I can tell almost all the water delivery services in HK use type 7 or PC bottles which are supposed to be among the worst.
The only exception is the Watson's 12L size, so if BPA is a concern that is your only option for delivery.
Also I don't know if / how it matters but I think that Watson's uses steam distillation while the rest use reverse osmosis filters.