I can't honestly vote for either option because each one creates a different 'feel' for the game.
Known encounters makes the gameplay flow better as there is no interruptions, however, it makes the encounters more 'safe'. Basically, known encounters are just that: known. There is little tension before the encounter, you know there is an enemy at that place, and you can choose how you want to engage with that foe, if at all. Simple, and gives lots of control to the player.
However, sometimes that level of player control could be detrimental to the game as a whole. Random encounters create a more tense atmosphere where a monster can come out of nowhere and eat you alive. If a location is deemed dangerous because it's filled with dangerous monsters that hunt people, the encounters there work better random. In fact, there have been entire sub-genre of RPGs that thrive on this level of tension: the dungeon crawler. Give the player a winding dungeon (or forest, or whatever) to get through with limited supplies, combine that with monsters that come out of nowhere to attack them, throw in a dash of surprises in the dungeon, and you get a game that thrives in part *because* the encounters are random. Instead of planning how to fight each encounter on their own, the player plans out whole dungeon runs. Perhaps the first time they stick their head in to gauge the strength of the foes, and the next they push in as far as they can, until they run out of resources or the player encounters a monster that they weren't expecting to encounter. It's one of my favorite types of RPGs.
That said, random encounters work best if the monsters generally are enough of a threat that the players might lose the encounter... or at least use up resources that are limited. Any game that you can just press 'A' to win, or has little emphasis on resource management, will probably now be a good fit for random encounters.
