Well, the RNS510 doesn't accept full postcodes still and you can't hack them to display speed cameras, there's no bluetooth support either and you need an MDI interface to get ipod/mp3 track names.
I agree the chinese version should be cheaper (I've seen them for less than this), but the 510 still isn't *that* good IMHO.
[an MFD2 owner who has just got TMC working and doesn't want to spend any more on the audio front]