What speeds (line rate and max attainable) is your HG612 reporting at the moment?
The general opinion seems to be that you can get a slightly higher sync with the Zyxel, but if (like me), you're already getting the maximum 80/20 out of your line with the HG612, there's no reason to replace it.
You'll also need to replace the firmware on the Zyxel if you want to continue with a 1500 MTU.