Depends on the speeds your ISP provides and the devices you use on your home network. If your ISP is not providing you anything over 500 mbps and you're not using a lot of devices or constantly streaming across several devices all the time then the 500 should be sufficient. But if have speeds more than that and you and everyone in your household are always streaming for long periods of time then the 600 would be better but the 500 will be sufficient too. 600 is a good way to future proof yourself.
I have the 500 and at any given time I have 14 wireless devices and 1 wired device. The heavy streaming devices are iPhone, iPad, Xbox One, Galaxy S7, Apple TV and a Smart TV. At any given time 3 of these devices will be streaming something at the same time. 500 works great for me. I also have a wireless camera, 2 wireless thermostats, printer, security touchpad, 3 laptops and chromecast. Those devices aren't heavy on download data often and my setup handles all of these devices perfect. I also have a nighthawk router which is more powerful than the N600.