The most important difference is nightlife. If you want discos and singles bars stay in Fira. If you don't care about anything other than a few quiet bars then Oia is the place for you. If you think you'll be spending much time at the beaches you might want to stay in Fira because that's where the bus hub for every beach is located, although you can get to the bus square in Fira from Oia too.
Everything else that Fira has can be found in Oia--shops, great restaurants, hotels, galleries, boutiques, and the incredible view.
Oia is famous for the sunset, but you might be able to watch the sun set in the sea from Fira with an unobstructed view depending on when you plan to be there. Part of the year it sets behind the island.
Walking from Imerovigli to Fira is the easy part. It's getting back after a full meal or a few drinks that might pose a problem. If you're fit you can walk between Fira and Imerovigli with little or no problem. Imerovigli is uphill about 1.5 km from Fira. There are two walking routes, one is up the main road from the Fira center square and the other is a cliffside path that also passes through Firostefani. The bus is another option, leaving the bus square about once an hour until 10 PM. There are also lots of taxis in Fira until about midnight.
are there busses that go often between fira and oia?
Have a look at this:
click on "Timetable"