The only difference you will get will be on your local (home) network if all devices have a GB network card/device to get speeds of 500mps - but having said that, in real life you wouldn't really notice the difference anyway as it is all so fast even on 100mps.
True, if you get 500megabit/sec. HomePlug is not comparable to Ethernet, its speeds are similar to 802.11 WiFi.
On G wifi which has an advertised speed of 54megabit/sec, you can in perfect conditions get around 22megabit/sec data transfer rate. ie, nowhere near 54. Same is true for Homeplug.
Many HomePlug 500 devices have only a 100megabit ethernet interface port, so will never achieve any faster throughput than 100megabit of 'ethernet' speed.
However the 500 protocol is better over the mains wiring when you have to cross between ring mains, e.g. between floors, than the 200 and the earlier standards. So I would always recommend 500 protocol, even if the LAN interface is 100megabit. You're more likely to get higher throughput.
As always test yourself, and don't rely on benchmarks, even on the same ring main.
500 devices can also act as repeaters, so a single 500 device in the front room connected to a games console or Sky+HD box can improve the connection from the office (where the broadband router is) through to upstairs - in at least two installations I'm aware of.
James BT Infinity 2
19/09/2012 - Sold 42/6 - Getting 46/8 - Sync 50 / 9 Mbps @ 470m approx
14 years of broadband (ntl: cable to BT FTTC) - Router: Asus RT-N66U - Modem: Huawei HG612 speedtest