Of course, the original Apple upgrade means that you get two 4GB units with your computer, rather than having the 3rd party units AND the 4GB RAM that is included in the MBP. You can sell the RAM easily, so in the end the 3rd party upgrade route is even cheaper.
3rd party memory also comes with warranty. Installing 3rd party HDD/SSD and RAM does not invalidate the warranty on other parts. The memory upgrade is just a few minutes, while the storage can be replaced in a couple of hours by either cloning or doing a clear install.