is not going to have the same battery life as the larger capacity iPods, even after updates. The 60/80 GB models are thicker than the 30 GB due to larger battery. I've owned both 5th and 5.5 gen 30 GB and there is little difference in battery life between the two.

So, if you must have that kind of battery life, go for the 80 GB. Even if you don't need the capacity, you could use the extra as file storage.