Less RAM and an SSD vs. more RAM and an HDD?
It all depends on what type of usage your rMBP will be subjected to. It does help with the SSD and just cutting it off at 8gb(I had a similar setup but with the earlier 15" rMBPs) if you're trying to save some money but make sure you have enough storage on the SSD(you'll run out fast). Not to mention 2011 RAM speeds are significantly slower than today's.