If I were buying one of these new systems (and I just might) I would strongly consider the 16GB version.
For me it comes down to risk/reward. I hang onto my Macs typically for at least five years. Often longer - for me the cost of such an upgrade over my anticipated lifespan is a pittance. It's almost ridiculous to quibble over by comparison. $200 is far less than the cost of swapping an entire machine.
If you routinely change your machines out every two years, then obviously the risk is greatly reduced. If you make the wrong choice you only have to live with it for two years before you will be able to correct it.
It's like insurance. Will you pay out more for premiums vs. the value you get?
Speaking of insurance, as I stated earlier if it came down to AppleCare or the RAM I would pick the RAM every time. Then probably still score a cheap Applecare serial number off of eBay
You can also get the 7 core GPU with 16GB - that knocks $50 off the RAM upgrade. If I was counting every dollar I'd start there.