I predict, without reservation, that 4E will be a fundamentally more successful business based on what I've seen. I think a lot of people who are bent out of shape about what they have heard so far are going to buy the rules, like them, and convert their campaigns. I think others will be pulled along by their friends and will grudgingly go over, and find out that they like the system.
I am trying to take all of the support for a 3.5 "legacy" offshoot with a giant grain of salt. I think the majority, perhaps a sizable majority, of the existing audience will convert within a year or two. I think a lot of people who are steadfastly posting against 4E on EN World are going to be excited as hell about the game in two year's time. A system can change a lot without really making an old RPG product worthless. So as long as the rules "feel like" D&D, people won't feel like they've wasted their investment. I think Wizards knows this, and I think they've likely made "make sure it still feels like D&D" their #1 design goal, their "golden rule," so to speak. And these guys are really good game designers, so I trust that they're on the job.
--Erik