Depends upon what you will be using it for.
For gaming 8GB is plenty. For common daily use, you wont notice a difference. For running many VMs/many apps at once, itll help you (although i doubt you will need it).
All in all, i doubt you will ever see an improvement from 8 to 16. Anything above 4GB and the returns are small (unless running apps that require large amounts of Ram). If you have to ask about RAM, theres a 99.9% chance you dont need more than 8GB. I render/VM/game/fold/program in my computer, and ive never been over 5GB @ one time. lol
BTW, I build computers, and 16GB of 1600Mhz RAM runs $100 or less in DDR3 form. Just goes to show how much Apple is $crewing you over. I hate Apple....