i was a learn-to-swim teacher through a school before DS. (but did private groups on weekends aswell) and i honestly think that kids respond better in a group. had quite a few one-on-ones and they were more inclined to 'muck around' because there was no real break (iygwim?) with babies, I also think that the mums do better in a group. especially if you all have to dunk your babies, at least your not the only one!

would always recommend a group. then the babies can see other babies too.
good luck! learning/teaching to swim is so much fun