I don't have 2015 nor 2018 deluxe. The biggest change if you do rendering, is the inclusion of Lightworks as a default rendering engine, instead of RedSDK, (RedSDK is now a plug-in). If you purchased the Lightworks plug-in for 2015 then you won't notice any difference, if not then rendering will be quite different.
The three links will show new and improved features in 2016/2017/2018, All I can suggest is having a look at the features, (in the deluxe columns), see if there are any you think will be useful. Then download the deluxe trial, and try out the features you thought about.201620172018