The iPad is certainly the 800 pound gorilla. It has the most software for it, and is the most compatible with accessories.
I’ve played with a few low-priced units and have found some better than others. Units like the Coby Kyros are inexpensive and work quite well. The short-fall for them is that they are usually built using the open-source versions of the Android operating system and aren’t officially blessed by Google. This means that you won’t be able to run some of the software on them that you’d want to, as Google restricts access to it from these types of devices. I got around much of that myself by signing up as a Google developer and registering the tablet under my developer account — but that’s kinda nerdy.
Unless you are willing to pay somewhere around $400 for a tablet from a cell phone carrier, you are going to run into some bumps in the road. The various tablet manufacturers even make different models for direct sale verses being sold via the carriers. You should probably check with retailers like Best Buy, Radio Shack, Ritz Camera, etc. to find something that will meet your needs.
We’re still in the “buyer beware” mode in the tablet arena, which brings us back to the iPad. Certainly the best bet — although pretty expensive.