Spark is targeted at people interested in all-day activity tracking such as counting steps, active minutes, sleep and calories burned, as well as sport activities: it has a design that is fit for wearing all day every day. You can find it in Consumer Electronic stores.
Runner is targeted at runners: its design is more fit for running and other fitness activities, with brighter colours. You can find it in sports and running outlets.
The watches have the same hardware and software.