According to Ionic Framework Officials, Ionic 5 is 2X faster than Ionic 4, it has lots of design related improvements specially for latest version of iOS like Dark Mode, Segment, Header, Large Title, Small Title, Swipe to Close Modal, brand new Icons and Animations.
But the question is, should I migrate from Ionic 4 to Ionic 5? Ok, before you do so, you should estimate the size of your app and how much changes you have to do if you migrate to Ionic 5. Because, if you have a app with thousands lines of custom code, lots of third party plugins, it may break after migrating to Ionic 5 and fixing all of them become a gigantic task which does not worth to achieve the new features of Ionic 5
One of the most efficient way to estimate the amount of task you need to do after migration is, create an Ionic 5 project and port your existing Ionic 4 project into Ionic 5 projects. Then you manually install the plugins you used in Ionic 4 project and finally try run the newly ported project. You will get all the errors you have to fix in terminal. Once you fixed all of those errors, you can check your full app and see if it is functioning properly.
You may heard that Ionic 5 deprecated Ionic 4 Events, on-segment no longer triggers an ionSelect event. You need to use ionChange event to trigger an ion-segment. Many of Ionic 4 icons does not show up in Ionic 5. You will find many other aspects like this if your project is really a big one.
So in this way, you can easily estimate how much rework you have to do after migrating to Ionic 5.
If you are craving for the performance upgrades, new popular dark mode compatibility and design improvements for latest iOS versions, you really need to consider migration to Ionic 5.