1. You don't need to play Myst to understand Riven. The plot revolves around characters who are introduced in Riven, and the plot of Riven is only mentioned once in Myst. You won't understand some of the back story, and there are certain references to Myst It won't ruin the game for you if you don't play Myst, but I would suggest that you play it first anyway.
2. Myst-
Pros- Intriguing story of betrayal. Puzzles are difficult, but the solutions are straightforward and easily accessed. One does not "die" or lose until the very end of the game. And chances are if you have made it that far, you will know what you need to do in order to finish. The music is great. Two characters have their own themes, which are repeated in different forms over your travels. That gives a sense of connection to the locales you visit, rather than them being completely separate journeys.
Cons- Though the graphics are good for the time, they look a little unconvincing at times. The same goes for any videos. The end of Myst seems unfinished. After completion of the story, you are allowed to continue playing; visiting all the places you were able to before. It isn't an error or programing goof, because they recognize the fact that you will be allowed to "explore at your leisure".
Riven-
Pros- Takes everything Myst did, and does it better. The story becomes far more engaging, the characters are more vibrant and believable, and the graphics are simply stunning, even after all these years. The puzzles are more difficult this time around, and they take a little bit more searching to solve, but are still logic oriented. Some require a touch of abstract thinking. The music creates a specific mood, much like Myst, only more so. Little details abound. You can see ants crawling on the walkways, beetles will fly away if you get too close. Each opening door has an animation of the door opening to add realism, rather than "now it is closed, now it is open, let's walk through."
Cons- The puzzles could be perhaps a little too hard. Depending on what edition of the game you get, changing out discs could be tedious.
3. I prefer Riven. The game lasts longer, and it doesn't retrace steps like Myst did. It is a far more engaging. The characters have purpose, and create a connection with the player. But don't just buy Riven, buy the whole series and enjoy the adventure from the first step!