Really depends on what I'm doing and what's going on around me.
If I'm trying to write a song or put together a set list, generally I'll work through it with my acoustic. For me, my acoustic (being a Taylor 614) does the simplest, most direct job of translating emotion in the playing into emotion in the sound. It removes all the other complications of the guitar settings, effects settings, amp settings, etc. and lets me just focus on the music.
When actually playing live, it always depends on whether we have a 2nd guitarist (on electric) or not. If we don't, then I'll play the whole thing on the acoustic. If we do have the other guitarist, then I'll switch between them, but probably spend more time on electric.