First of all, Appguard needs to be configured smartly if you want maximum protection. But ERP is configured fairly well, right out of the box.
Second of all, the big advantage of adding ERP to your security config, even if you already have Appguard, is that ERP gives you fine-grained control of vulnerable processes.
As an example: if you block cmd in Appguard, that's it, all executions of cmd will be blocked, even the executions that you want. But in ERP, you can set cmd to alert, thus you can allow the cmd executions that your software needs in order to be happy and healthy. The same is true of rundll32. In ERP, you have fine-grained control over it.
On the other side of the coin, there a lot of things that Appguard does, and ERP can't. You can block certain dlls, for instance. Even more important, you have Guarded Apps.