Its essentially the same game, but there are three distinct differences:
- gamplay: Myst me is an old school sideshow point and click. Its not as immersive maybe as real-time but it somehow feels more ambient than realMyst. Realmyst will let you walk anywhere that you want (if you've played portal then it will be like how it is before you get the portal gun) but this sometimes feels awkward.
-Graphics - Myst me has pre-rendered graphics and realmyst has real time ones. Myst me has old-looking textures that don't look "realistic" by today's standards, but it still has a weird photo-realism about it simply because the edges aren't jagged. Realmyst uses real-time graphics which look much more "real" in terms of today's games with textures and etc but it also looks very "cg"- jagged edges, occasional texture errors. Personally, neither of them end up feeling real the way Riven does for me so its really up to what you think looks more real.
-New Age- realmyst includes a small age at the end of the game which Myst doesn't have. Its nothing real special except for that it looks nice and connects to Riven a little more clearly. It is also mentioned in Myst IV.
Personally, I prefer realMyst because the story takes precedence for me and realmyst has that one age, but other than that its really down to personal taste. If you're comfortable with games like Portal then go ahead and get realMyst but if you can you might want to buy the me just to see what its like.
Either way, now adays Myst/realmyst is really just something you use to get to Riven, possibly the best game ever made. Play Riven.