Network performance tester might show that you have 5% packet loss or 3 milliseconds of jitter, but does that mean it would have a noticeable impact on your network performance and fall short of the SLA? Whatever tool you use should have a configurable screen that reports a pass or fail.
How well the router will perform when you specify a 150 Mb circuit and other specific loss and jitter parameters? The network performance tester has many more criteria that you can set.
A feature in network emulation tester is testing the configuration before it runs a full-blown test. This is extremely helpful because the network emulation tester reports why it failed, so you can adjust your test parameters and know if they're valid before running extended tests.