I think it depends a lot on what type of game you're making. As a player I prefer real time travel, but that's because I like the immediacy of it. I feel more immersed and connected to the places I'm going and I get a better feel for the distances between them. I also play a lot of action rpgs and since this is an almost necessary part of the experience in those games, I'm probably biased towards them. Overworld is great for giving a broader sense of scale and wonder, so it can be important for an epic game. I think both at the same time can be really great, like Secret of Mana, where you usually travel the slow way, but as the scale of the story ramps up you use the overworld map more and more and get an idea of just what some of these places look like in a wider view. Or Chrono Trigger, where the ability to travel overworld gave a sense of how civilized the area was, since the safer areas had less "locations" in your way to getting somewhere else, but fairly normal places could still be dungeons. I donno, it's hard to say. They're both equally pretty great options, if done well and can be used together to do fun things. I think the more important question is what is right for your game in particular? What would reinforce the mechanics, tell the story better?