If you get a tablet, decide if you want a resistive touch (using a fingernail or stylus) or capacitive (use of fingertip) screen. Also if you go for a capacitive screen, consider multi-touch, meaning you can zoom in and out with two fingers (like I've seen on some iPad commercials).
My experience has been with Android tablets. I've had a couple of old ones with Android 1.6, a couple of newer ones with Android 2.2/2.3, and now one with Android 3.2 (it was 3.0 when I bought it, and has been upgraded twice). Plus two different Android phones. I love them all. But if I was buying now I'd wait a few weeks and go for an Android 4 model, which will start coming out next month.
There are note apps, reader apps, planners, they play movies and music, have games, many can play flash, etc. Most have bluetooth, many have 3G capability, and all I've ever seen had wifi. Basically most anything you would need for a mobile computing device.
But if you plan on taking notes, I'd get an external keyboard. There are keyboard/case combinations that don't cost that much. Just make sure you get one that will work for you (i.e. if you get a bluetooth keyboard, you have to have a tablet with bluetooth; but there are many that will work with bluetooth, and many more that work with a USB cable plugged into the tablet; also making sure that a 7" case is acquired for a 7" tablet, or a 10" case for a 10" tablet). I have 7" tablets only (I think the others are too big) and have two different keyboard/case combinations that I truly love.
Sorry, can't compare with the other sorts of tablets since I've only seen iPads on display, never in anybody's hands, and I've only seen Windows-based models for sale on Craig's List, not in stores or in use.