Welcome to the forum Mark. Great place, great people with loads of good advice on here.
With regards the RNS, the general rule is buy the newest you can afford. Have a look in the Sat Nav and Car Audio section here:
http://www.mk5golfgti.co.uk/forum/index.php/board,19.0.htmlLoads of info there.
What year is your car. It's generally the pre 07 cars that needed the gateway changed if running the newer units, but mine is an 08 and I had battery drain, so got a Gateway of Eddie-NL on here. Ed will be able to tell you best. The aftermarket units have their place, some great units out there, a couple of them, Pioneer and Kenwood even show the optical parking display and climate on them with an additional module, but if it's overall integration you want and OEMness, then stick with the RNS, great units.