& here comes Tell

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Thanks for that Freddie I forgot about the DVD ... will edit it in

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I did order it and I have got it.
The Seat information isn't clear about what it is and is inaccurate. First it's not a hard disk drive it's solid state, like SSD. Seem to recall 64 GB. 10GB are spare and you can use as a "Jukebox". It comes with the Gracenotes database which I think does your album art but I do myself. Certainly when I put an SD card in, it pops up Gracenotes.
Because it's all solid state it's faster than the mapping running off the SD card, but nobody knows how much - this is what the German VW boards say as well. It uses 3D maps so in town buildings will be given the height in the 3D projection so when you are driving in town you get a better idea of where you are. I have caught site of it doing that as one drove through Lille. The standard system doesn't do that, not to that extent.
It has wifi, this is essentially so you can pair it up to a media player rather than the cable. I've experimented. This you can only do with an Android as I understand since iPhones won't act as Media Servers. It will also sends out a SSID but that drowns out other SSIDs so I turned that back off again.
Since you have the maps in solid state you actually have two spare SD cards so I use both mine for my FLAC music collections that I've been Flaccing since June... not there yet. At 193GB on that so far, another draw of minidiscs to go... Split them down the centre by alphabetic letter into two 128 GB cards.
The unit looks identical to the standard unit. The extra stuff is hidden behind the CD / SD unit in the glove box (it's actually a DVD drive). You only know it's a Navigation Plus when you press source and you see JukeBox shown although obviously the service menu will tell you. It comes with the maps on the drive so no issue with the card getting lost.
Edit..oh I forgot it plays DVDs and video files held on USB / SD if stationary or you can hack the car's system via the device on the other thread or other devices, adjust the speed limiter and break the law. It does have a nice touch thou if you buy CDs abroad whilst travelling rather than playing them off the DVD / CD player you can copy them to the Juke box so you can keep the CD pristine for converting to FLAC when you get home.
I would say it is a luxury. German folk like them. It's the equivalent of the VW Discovery Pro although what they have added to it is slightly different. It's a MIB2. You don't get VW Car Net but then I don't think VW owners get that in the UK in any case. You will see that people specify them on the German Ateca boards.
In the UK the Xcellence comes with the standard unit priced in, on the continent they are priced in differently. I think it's over priced in the UK and falls victim to how Seat haven't got their pricing right and bundle the standard unit in, effectively you pay for the navigation unit again when you buy the Plus unit. There should just be an upgrade cost, not walloped with the full price of the enhanced unit some of which you have already paid.
The other thing to consider is if you are one of these connected people that wants to run their maps off their Android and iPhones and think that's better there is no point in spending the extra for the Plus unless you want the two SD slots the Juke box for music storage you got the phone for that. A lot to be said for built in car navigation over the mobile device based, but saying that when I'm acting as the navigator in foreign parts I'll have them all switched on and look between them. Handheld GPS app running a track over where ones been and the track to follow, Google maps running on a tablet and the car's system doing it's bit. Car systems give you better lane guidance and don't jump roads as the navigation signal drops.
It comes back to what I said earlier about the Navigation Plus being a luxury and whether you are prepared to pay for it. For me, I don't have to flip SD cards when I'm going through my music collection which is all lossless and don't keep duplicates in MP3 / MP4 etc, that's the tangible benefit I have realised, speed I'd need the standard one to test against.