Both my Garmin 3790LMT and Volkswagen RNS510 are fully up to date. To give an example of how they differ, the RNS often gives the instruction "Stay on the AXX for a very long time" - it's not even capable of giving accurate measurements, slightly different from the precision of the Garmin.
During the lifetime of the Mk.6 Golf, VW dealerships offered a head-unit incorporating a Garmin Satnav with the same features are the nüvi 3790T, this was a was better and more versatile unit that the Volkswagen RNS510.
I don't have personal experience of the Volkswagen Discover unit, nor am I likely to at the ridiculous price Volkswagen are asking for it - bearing in mind that a Satnav seldom makes much different to the resale value of a vehicle, it just doesn't make sense to buy it when superb navigation is available in a stand-alone Satnav costing £2-300. Also bear in mind that with the Garmin unit you can plan more complex routes on PC/Mac and transfer them to your Garmin, something you can't do on most Satnavs. It's even possible to set up routes with photo POIs etc.
I totally echo what Essjay says about POIs etc. - I found my Garmin nüvi 3790T amazing in Europe last year with such things as lane directions and even photos of individual junctions showing which lanes to take etc.
Mcmaddy obviously knows best and has money to burn, but I don't. The maps are one thing, but the features of a particular Satnav are something else - give me a stand-alone by choice.