The QNAP NAS devices use mostly the same firmware, if you like one you'll like the others. Features that actually separate the models come in the hardware and the (storage) expansion options. We already knew the firmware is easy to use and full of features, there wasn't a doubt there. So what kind of actual improvements does the TS-509 offer?
The TS-509 uses far more powerful hardware which improves performance over the previous generation a lot. This basically proves the hardware has been the bottleneck all this time. So while the TS-509 doesn't really offer any new features, it offers a lot more performance. Additionally, it offers slightly more options to connect external devices.