Depends on use of course, but I'd say that comparison is the most accurate I've seen. Certainly, more so than any of the others I've seen including the often sited iPad v. MacBook one.
That said as an Air or iPad likely wouldn't be ones powerhouse computer (though either could possibly be primary one), I'd rather have a iPad over an Air for consuming purposes.
And I don't see the tablet as a "third channel" but as a eventual laptop replacement for most, the way the laptop has become a desktop replacement for most. Probably the same for iPhone OS over full-blown OS X. Eventually, the need for a laptop, like the need now for a powerful desktop, will be specialty uses. I suspect this would have been further along by now if MS did any real GUI innovation. Will be Apple and/or Google now.