Is there a difference in the build between the Late 2014 & Mid 2015 27" Retina 5k iMacs?

I ordered a 27 inch Retina 5k iMac a week ago at my local Apple Store with the following specs:

4.0GHz Quad-core Intel Core i7, Turbo Boost up to 4.4GHz

8GB 1600MHz DDR3 SDRAM - 2x4GB

512GB Flash Storage

AMD Radeon R9 M290X 2GB GDDR5

As I wanted the 512GB solid state drive the salesperson at the store made me do an online order as they only stocked the27 inch Retina 5k iMacs with the i7 processor in the store with the 1 TB & 3TB Fusion Drives.

Well my iMac arrived at the store for pickup yesterday and when they scanned the bar code it said it was the Late 2014 model and not the Mid 2015 model which is the current model and I was expecting to receive.

Is there any difference in the build of these two models that I should know about? The salesperson at the Apple Store said because it was a special order with the solid state drive it was showing as Late 2014 but was exactly the same build as the Mid 2015 model which I don't totally agree with or understand. Usually Apple makes some changes/improvements in the build in their newer models.

Also if I ever go to sell this iMac doesn't the build date of the machine affect the resell price?