*shrug* Depends on the use to which you're putting it. If you're a media maven and Just Have To watch streaming videos all the time
, then either an iPad or a top-end netbook with killer GPU. If you type a lot and have been typing for a long, long time on regular keyboards, then a netbook/laptop is the way to go.
Personal preference/need, really. I have a lot of gaming PDFs stored on my wife's netbook, which we take to RPG nights. While yes, the "short screen" is something of a weird thing at first, it's no better on anything that doesn't have a portrait-oriented screen that's bigger than the pages in your RPG book, and it took about 5 minutes for me to get used to the scrolling and such. So while it would be nice to have a full-page, portrait-oriented screen (and the iPad isn't quite a full page that I've seen, but it does have killer resolution and clarity to make up for that partially), it just ain't necessary as such.
Plus, when it comes down to it, my mobile computing needs aren't big but they're very specific. I RARELY find myself in a situation where I don't have a horizontal surface (table, desk, etc.) nearby and need to hold my computing device up, and when I do have said horizontal surface, I prefer the ability to adjust my screen so it's perfectly perpendicular to my direction of view, which can't happen with a tablet without actively holding it up.
Don't get me wrong. I VERY much like the iPad for what it is. But what it is is an expensive toy. And it doesn't fulfill the requirements of the OP at $500.