Learning D&D

Is it better to learn D&D (or any roleplaying game) from an experienced DM, or a brand new one? With a group of entirely new players, or a group where everyone but one or two people basically knows what's going on?

Is it better to learn to DM as your first D&D experience, or after you've been playing it for a while? With brand new players, or experienced players? Is it possible to learn to play D&D as the DM, but with experienced players?

By better I mean--which is more fun? Which is more likely to keep you in the hobby for the long run? Does it affect playing style? DMing style? DMing skill?

How do most people learn to play D&D? Are there differences between how D&D is learned, and how other games are learned?

What effect does already knowing another game have on the learning process? Does it have any other, longterm effects?

Probably some of these questions are answered by market research. I know that Wizards published a study of the player population and its various habits in the run-up to 3rd Edition. I assume they still do market research; do they still publish it? Do they have plans to publish it?