500 vs 550

[Hookster]

Hi can anybody tell me what advantages the 550 has, if any over the 500? I have bought a fully guaranteed refurbished 500 but I've seen somewhere on the unit (can't find the page again) '550' and when I connect it to Road trip it comes up as 550 in the connected device box.

It's not an issue to me and may be a bonus?

Thanks

Hooky