The thing is with the Nav Pro yes you get a 64GB SSD drive but you can buy 64GB SD cards quite easily today and that will hold the same amount of music (obviously). SSD is very fast at loading data but an SD card loads music more than quick enough not to notice any difference. So that side, in my opinion is pointless.
You can watch DVD's with the Nav Pro, I can on my RNS 510 also, how many DVD's have I watched on it? 0. So again, a feature that I wouldn't use.
Bigger screen and better resolution. Fantastic but again I can only see this being a benefit with the maps and rear view camera. Why do you need a bigger, higher-resolution screen for switching radio channels or going through the settings.
Now if it had personal wifi hotspot it would be a totally different story for me. But I think you gain very little for an extra £1,000 which let's face it, is a lot of money.
Yes it will be a while until these are available and cheap on e-bay but they will be available eventually and if you really miss having a bigger screen then you could probably upgrade quite easily and cheaply at a later date.
Of course it's down to personal preference really, I just think people are over-looking the most expensive option a little.