They are both important.

Lvl 2 Medicine will keep your villagers healthier of course.

There is something you can do if you have lvl 2 of science tech and lvl 2 of culture tech. I won't say here because it is a spoiler but if you are desperate to know you can read about it in the detailed spoilers guide by LadyCFII here:

Detailed Spoilers Guide HyperLink