Mapped.
Most definitely, mapped roleplays are my most ideal place.
While both have their positives, it's been my experience that you're more likely to have a consistent, long-running storyline and active members if someone puts the work into keeping a mapped roleplay up and running. It does have issues with getting the map to everyone, and of course keeping a lively member-base, but I really like the closer, more personal feel of a private map. It's more relaxed, and more than likely, people will read your posts with more attention, which really develops a nicer sense of presence per character, as well as mutual respect for each roleplayer.
That's not to say mapless RPs are terrible, though, since they're much faster to join and they're almost everywhere. It's more spontaneous, and I really enjoy the wide variety of them all-- my main complaint, though, is that a lot of people don't take them quite as seriously. They get distracted by the various stimuli around the public maps, and they're less likely to give a derp about reading posts other than those that are directly involved with their character, and the super-fast pace can sometime lead to a degradation of detail and, occasionally, artistry (this is a partial reference to the ratio of literacy to illiteracy).
And so. Those be my thoughts. I'll roleplay anywhere, but usually I end up preferring mapped roleplays in the long run.