RNS315 and RNS510 are both touchscreen and both store the maps on internal memory (RNS315 on internal flash and RNS510 on a seperate hard drive). RNS315 has a smaller screen (5") while the RNS510 has a larger 6.5" screen. RNS315 can be had with Bluetooth built in (but not all of them so be careful if relying on this as a needed function), also may or may not have DAB, and may or may not have the rear port for the reversing camera. Only way to tell is to physically look at them and check what's there. RNS510 may or may not have DAB but will have the rear camera port. RNS510 has a dvd drive rather than the cd drive the RNS315 has. RNS510 uses hard drive space for music storage, up to 18Gb or so, and can also have speed cameras added to the maps function.
The RNS315 boots up faster than the RNS510 (like 10 - 15 seconds or so while the RNS510 can take 50 seconds to be fully functional). That's not really an issue as it boots to music more or less instantly but navigation can take 30 - 35 seconds before ready to use.
As regards buying one, there's a plethora of old RNS510 units on Ebay at the moment (generation 1 units with poor quality screens and flakey hardware) so avoid anything without a "Star" button on the top right hand side as the Star button units are much better.
Expect to pay anything up to £500 for a DAB RNS510 or £375 ish for a non DAB RNS510.
Expect to pay anything upo to £400 for a fully loaded RNS315 or £200 for a basic non Bluetooth, non DAB unit.
You'll need the security code for any unit you buy. If the seller doesn't have the code then don't buy it. You can get the code for a tenner or so off an Ebay seller (using the serial number of the unit) so why doesn't the seller do this?
You'll need a GPS antenna which cost about a fiver off Ebay and if going for DAB then expect to pay £60 - £70 for a genuine VW DAB antenna or maybe a tenner for a cheap and nasty stick on the screen one.
Maps updates are expensive for the RNS315 and you may have problems if you do update yourself as later maps versions are not compatible with earlier units. Maps updates for the RNS510 can be had online for free if you know where to look and how to burn a dvd.
The car and the unit will need coding with VCDS to make everything work correctly. Anyone telling you it's plug & play is talking out of their arse. Most features might work, but things like steering wheel buttons or in dash display may not work without coding.
I sell a few of these, and I have a couple coming in this week if you're interested. They'll be fully updated and tested, and as cheap as anything on Ebay with aftersales support.