There's also a size/flashiness issue. If I'm going on public transit or waiting somewhere that the whole "look, I'm holding a shiny device that's easy to pawn!" thing may be a bad idea, I bring my kindle. Or if it's likely to rain or I just don't feel like dragging around the ipad and a cleaning cloth (what's with those smart cover streaks, anyways?).

Otherwise, I take my ipad. The addition of news reading (the economist and reuters apps are fantastic) is why I usually bring it instead if I can.