they are two different batteries, from my research, the mid 2009 15" MBP has a 73-watt-hour non-removable lithium-polymer, and the mid 2010 15" MBP has a 77.5-watt-hour non-removable lithium-polymer. They appear to be the same size, but only question is will the mid 2009 MBP accept the 2010 battery and its larger capacity. If it is compatiable, i would get the 2010 battery hands down. I mean if your gonna pay the same price for battery replacement, might as well get the one with a slight more improvement. but knowing apple, they only want you to use the battery that it was sappose to use for.