Hello I am trying to decide between the 650i convertible 2012 or the 640i conver

A 7 to 8 year old BMW will likely cost you some money in maintenance and repairs -- even with a warranty in the short term. You just need to know what you're getting into. It doesn't mean they are bad cars, it just means that any car with some age will end up needing something in time. BMW's tend to have pretty extreme repair costs, so that kind of comes with the territory. Good luck.