Apple RAM upgrade is twice the cost of OWC. But if you wanted the 16GB, you'd be better of getting it as a custom build from Apple rather than dealing with a user upgrade.
I can't believe if I wanted to replace my iMac in 2013 I would be almost forced into moving to the 27" model with a screen size I really don't want with an extra price I don't want.
I was in an Apple retail the other day and they had two iMacs on display. The rest of the store was all IOS.
???? It's not clear from the OP whether he is buying the 21'5" or the 27"- a 27.5" there is not!!
If it's the 21.5", then yes, get the Apple installation on buying it. If it's the 27", no, buy from Crucial, OWC etc, a 2 minute juob to install and no loss of warranty.