Albisrieden is on the flatter side of the Limmat river, which is an advantage if you like bicycling. Also, as you can see on
this map, Albisrieden is closer to the center than Zürich-Affoltern. Although I know a few nice restaurants in Albisrieden

, I would argue that Zürich's nightlife happens mostly outside of Albisrieden and Affoltern. Affoltern is a bit closer to the next motorway but it really depends in which direction you have to leave the city, because Albisrieden is near the city border, too. Affoltern is a part of the city that still grows. In the next eight years, new buildings will be made and the tram connections improved. Albisrieden on the other hand doesn't have a brand-new townscape and is already connected by tram, not just with busses and trains.
Personally I'd prefer Albisrieden, but only because of the more central position and the relative flatness.