Osaka definitely has a better nightlife (though Kyoto has one too)
The other pro for Osaka is that it is a bit more central location if you are also planning trips to Kobe and Nara. Though you won't be at a disadvantage by staying in Kyoto, which is also convenient to those cities.
Kyoto's big plus is that it has way more stuff to see in terms of historical sites, temples, shrines and traditional Japanese stuff. Osaka really has none of that worth mentioning, though it does have a lot more shopping and other big city stuff.