When it was time to get that 4K retina iMac, I only had money for one built-to-order upgrade. It was a tough decision. It was either leaving the internal 1TB hard drive and choose to have 16GB of RAM instead of the base 8GB, or choosing a 256GB internal SSD and keeping the base 8GB of RAM. Since RAM in that 21.5-inch retina iMac is only upgradable at purchase, not down the road, I opted for the 16GB of RAM and staying with the hard drive. My reasoning is that I can upgrade to an external Thunderbolt SSD at a later date when I can afford it. RAM is not upgradable later.