Brain Age 2 starts with more games and I seem to be playing it more. When I hand my player over to someone for them to give it a try I am recommending Brain Age 2 over Brain Age. But its just my personal preference. There isn't a prerequisite.
Brain age 2 is more value for your money in my opinion. It has very nice exercises plus a DS version of Dr. Mario. I played Brain Age 2 before Brain Age 1 and I consider 2 to be the better one. As a plus Brain age 2 comes in different languages. Brain Age 1 is English only.