it really depends on what exactly you are doing, or want it to do.
Netbooks:
The nice thing about a netbook is that it has win7 on it, it runs all x86 software, with full driver support for just about anything that you could ever want to hook up to it... think of a netbook as a swiss army knife: No good for big tasks, but great for small stuff, especially when considered an accessory to a real desktop computer. The other nice thing is that you have the entirety of x86 programs out there to install. Real programs, not just cutsie little apps. It will not game all that well (though bloons tower defence works... what else could you need?), but for everything else it is 'adequate', while being able to hook up to a real monitor, keyboard, and mouse when you need to. I have 2 netbooks, which I 'pimped out' with faster HDDs and 2GB of ram, and they run great that way! Stock they were just passible, but with a real drive, and some ram to pull from they behave better. Netbooks with newer AMD or Atom CPUs would run much better as they both addressed the one complaint I have with mine: the GPU sucks! I mean sucks so bad it cannot play HD video without droping frames. The CPU is fine, but the GPU on those old models is simply bad... but like I said, newer versions have addressed this issue.
iPads
the iUniverse can work pretty well. I will be up front and say I am generally not a huge fan of Apple products, but now that I have used some iPads at a school that I do tech support for, I do understand the draw. They are great for media consumption, and for canned productivity workflows (like textbooks and stuff), while offering a passable internet experience. Not having flash is an issue, especially with educational websites that seem to love the stuff, but there are generally ways around it. While most of the programs on the iPad are absolute crap made by people who just wanted to say that they have made an app, there are quite a few really good apps out there, so for just about every need there really is 'an app for that'... the problem is finding which ones are good. Also, they do make accessories to add a keyboard/dock to an iPad, it is just a limited selection, and because it is 'for the iPad' instead of 'just a keyboard' you will pay extra for it... but that is the general rule in the iUniverse, things work fine as long as you keep to the well beaten path, and so long as you have money to plunk down for accessories.
Android:
Android tablets are a mixed bag, and you definately get what you pay for. The cheap ones tend to have a lot of features for the money... but are easy to break. The expensive ones seems to do a lot better, but you are still limited by the OS itself, which cannot seem to pick a clear direction to move in, and is in a kinda wierd state of flux right now between phones, tablets, and looking to move to the desktop market. Like most versions of Linux, android suffers from market fragmentation, with just about every hardware maker providing their own software suites, interface, and hacks to get it to do something special. I have played with ASUS and Acer android tablets, and they seem fine, but as I have not purchased one I have not gotten to really know that OS all that well. But in general you can expect less (but more useful) apps, more supported hardware, and a host of interface designs to choose from which will be different from device to device.
Chromebooks:
The new Samsung chromebook looks sweet. I love Google Chrome, and using it as an interface for a linux distro seems very interesting to me. Sadly I have not had the opportunity to play with one, but for productivity work either a chromebook or a netbook are going to be your best bet for the price range you have set. On top of the hardware, you are also getting some of Google's higher end services free for a few years, which can be a nice perk. The only drawback is that Google has not really put a whole lot of weight behind ChromeOS. I think as a company they want ChromeOS and Android to somehow magically merge to provide a platform that covers phones to desktops, but they seem to be missing something to make it work. So the fear is that ChromeOS will be replaced by android in the future, or that it will simply be neglected in a corner in the future, leaving users of the product behind. But if it gains traction with this 3rd revision then maybe things will get better?
win8 tablets:
I know Surface is expensive, but if you wait a little bit, there will probably be a cheap tablet released in your price range. It may not be as sexy as the surface, but it will have a modern OS, real USB functionality with wide hardware support, Office2013, and a quickly growing library of apps. This would be the hands-down best direction to go as a student for the budget, but we simply do not know when such devices would come out. There were rumors that the new B&N Kindle would be the cheap win8 option, but that obviously didn't happen, so it would simply be a waiting game. I am expecting ASUS and Acer to make win8 versions of their current Android tablets, but it still may be a little while before we see a cheap one that you can plug a USB keyboard into.
hope that helps in some way!