Is the M3's battery life noticeably better than the M2's for everyday use?

You probably won’t notice day-to-day. The M3 offers maybe an extra 30-60 minutes under typical office work, and both are rated at 15 hours wireless web in real-world testing. If battery life is your absolute priority, the M3 wins, but if you already have an M2, don’t upgrade for this. The real battery killer in either is display brightness, not the CPU.