As a Wacom user for YEARS, I have to say that I would highly suggest Wacom's products. They aren't just easy to look at, they are technologically built for what they do best for artists, designers and photographers---they draw naturally. I have NEVER been a user of small sized tablets because I feel they restrict how much space i need to draw.
Here's a little pressure guide:
Bamboo/Graphire: 512 points of pressure
Intuos 3 (and assuming earlier versions too): 1024 points of pressure
Intuos 4: 2048 points of pressure
Crazy huh??
Recently, my Intuos 3 went to Wacom heaven, but I am upgrading to an Intuos4, I'm pretty excited to get it~ Though I still prefer the penshape of Intuos 3 because of the long grip. <3
Now, as for performance, like fataltea said, it depends on what you plan on using the tablet for. If you are an artist like me, the more pressure the better. Preferably, a tablet that has a working area of 6x8 is better than a 4x6, especially when you prefer to work large like me~ Generally, it doesn't matter what size it is but it helps you out a lot when you like to move around and like to work more freely/liberally.
I would suggest a photographer to go with graphires and bamboos because they don't need the pressure sensitivity. Typesetters and type artists don't need a lot of pressure either.
Artists, I would suggest Intuos 3 generation or upgrading to Intuos 4 for their smoothness and slickness in Photoshop or any other painting program. Being able to have absolute control is essential.
I don't know if you are on a big budget or a small one but I highly suggest to stay away from cintiq (from wacom) the first generation has a lot of bugs and dead pixels and has a shitton of wires/hardware that comes with it so until they fix these issues, go with USB hardwire tablets!
I hope this helped!