Mexico or Canada?

Thanks fist for those who answer my questions:

1).My parents are visiting us in U.S.They arrived U.S last month.Their B2 visa is six months valid and Mutiple entry.If they like to visit Mexico for a week.Do they still need Visa to reentry to U.S.A?I think no.But I like to make sure of it becasue of the importance.

2).I do not know which is better? Mexico or Canada? Or other countires nearby.