I use a MicroTrack II, have had it for nearly 6 months. I have not had any problems with the battery running flat. You should be ok for any normal length show. What's normal, I know. I have recorded the support and main act with no ill effect. If necessary you can get battery back up. I have an Energizer Energytogo back up. I bought it to take to ATP, for the weekend, but forgot to pack it, I managed nevertheless. It's to power mobile phones, make sure you get one with the right connector for the MicroTrack. The mini USB underneath.
If you're recording shows and you haven't done it before, I strongly recommend going out and getting some practise. Same applys to new recording gear, you've got to get to know it before the main event.
I was interested to see a recommendation of reformatting your CF cards after use, to avoid the controls becoming frozen. Something I will start doing after such an incident last week.
Good luck with your purchase(s)
jimcooke.
Using:- 2 x Sony ECM 50 mics> Microtrack II>8GB SanDisk Ultra II CF ( I usually use Core Sound Mics but there in getting fixed)