In general the ones that transcode a DVD to Divx are likely to shorten the battery life. The optical disc isn't spinning, but the tradeoff is the CPU (and possibly the GPU is it has decoding features) works harder and draws more power.

Do some benchmarks of your own to see what works best, keeping in mind the platform the system sits on should be identical for the tests due to heat managment, and a fan possibly draining more power.