Alternatively you can navigate to the Apple logo on the upper left side of your mac and select About this Mac. Then select the Memory tab and check the specifications for the RAM modules.
According to your comment, it looks as if you could do better with more RAM. If it was just launching slowly, then I would say the SSD, but since its switching apps also, I think more RAM would be nice. Unless your iMac ran a lot better under Mountain Lion or below, I really wouldn't downgrade.
BTW: Don't disable the user accounts. Just log off when you are not using that user account.
Since you have a spare SSD, you could also just connect it externally using an inexpensive USB to SATA adapter and install new OS on it and run some A/B tests. You won't see the full speed of an internal placement - especially if you use USB 2.0, but it's often enough to know rule out RAM or OS as a cause of slowness.
Best Answer
I've used Crucial website for all my Mac upgrades and never had a problem.
You can specify which model you want to upgrade and it will give you all the available (safe) upgrades.
In this case: Link
Alternatively you can navigate to the Apple logo on the upper left side of your mac and select About this Mac. Then select the Memory tab and check the specifications for the RAM modules.