tin has a weaker electron-phonon coupling than lead. So at room temperature, the conduction electrons do not get "decelerated" by the phonons as strongly as in lead. So they can move more efficiently.
Wow that's so cool (I've never heard of phonons before)!
I might not be well-informed, but I will try to understand why tin has a weaker electron-phonon coupling than lead if you are kind enough to share (unless it is too complicated to explain, which I hope it isn't).
Thanks a lot!