I'd suggest converting footage on windows. Mac currently lacks *good* native software, so you'd be resorting to wine or crossover to get windows software running.
The future native alternative is to use vapoursynth
but for the time being you'd have to build it from the source and it's kinda early to adopt it, I'd say. I'm actually tempted to hold off on the newer avtech (and actually basically am...) because with vapoursynth in play I'd have to make a newer one since I'd be able to write a mostly cross-OS guide, which would be a great addition to the site.