I went from a 2011 (which recently developed a weird intermittent power fault) to a 2018 model. The main difference is that the new one’s way lighter and thinner which is great because it comes with me on the bike to work. The battery is better, too. Oh, and the retina screen is awesome.
But honestly, none of these things are game changers – whereas an SSD in your machine will be. There are also aspects which are worse about the new model – it feels less robust (I don’t know whether it actually is, time will tell), and the keyboard isn’t quite as nice. If your machine works fine and does what you need it to do, I’d save the cash.