Re:
I'd say your friend is partly right. A decent modern full-sus or hardtail bike will seem years ahead of a similar retro Mtb... because they are!! For a lot of people the terrain they ride has changed, and modern bikes are best suited to this. Retro Mtb's are best suited to the type of terrain folk ride them on back in the day (which was basically anywhere, excepting modern trail centres.)
But both retro and modern are great fun, just in slightly different ways.
I'd say start off getting back into things with a retro Mtb, it'll probably be capable of handling the vast majority of whatever you can throw at it.
But when things get rougher, rockier, jumpier, and faster, then try a modern Mtb, you'll be surprised at how well they cope with things a retro Mtb just can't.