Most I know use a medium size Intuos 4.
In the Cintiq route, I'd go the 21", I know too well the 12" model.
If is for pixel art, some retouching, etc, nothing too serious, even a Intuos Small could do. Or a bamboo Medium size, however they call it, not the smallest size.
If you want more precise line drawing, like comics inking, etc, I'd recomend a large tablet. The XL (works like charm) is cheaper than the cintiq, is like 800, way cheaper than the 21" cintiq, and for those used to wacom tablets already, might be even more convenient. Other than that, just go for a L or an A4 variant (wouldn't recommend the Wireless model), that is, the "M" models. For inkining or just having good stroke control, I personally wouldn't go lower than L size. Always depend on the space available in your work environment, table... large sizes require large monitors, too.(no 17" ones. A 22" would be ok)
I have had diferent opinions about this matter, but this is sort of a conclusion after working with practically all Wacom products.
Oh, and yes...Wacom all the way, in my opinion. And most other artists I know think this way.
Edit: When I said I used many Wacom products, I meant I have used (at home or work place) heavily several flavors of Graphire (1,2,4), Bamboo, an old Wacom 1 A4, Intuos 4 Small(S), Cintiq 12", Intuos 4 XL. The XL is great if you need a lot of accuracy, but many get back to the shop as people don't think well how much space does it use and what means to handle a so large tablet. Still, my best buy till the moment. Intuos 4 Small(S) is probably the best bang for the buck of these all, but I would better recommend a Bamboo fun (while the keep it being the only "MEDIUM" size of all bamboos, all others are "Small"). This is my recommendation for the majority of users. The features you loose for not being an intuos4, do not worth loosing the larger active area of this Bamboo (6.2" x 3.9" the Intuos4 S, while 8.5" x 5.4" the Bamboo Fun (medium size)).
Really, the control on what you draw is so much better at larger sizes. Still, it has a size it can play well with keyboard and mouse placement. And sort of 30$ cheaper than the intuos4 small. For more people needing illustration accuracy, might be better to go for the Large Intuos4 (L) . The Medium size of Intuos4 (8.5" x 5.5") has same active area than the bamboo fun! So, you see how the Fun is a great purchase (unless you know you badly need the intuos4 features), specially with the big price difference between Intuos 4 M and Bamboo Fun. So I'd go for a Bamboo Fun, or jump to the Intuos4 Large (L).
The cintiq 12", don't do it. I did. With that money you get a ton of great hardware components. In my tests,my XL is way more accurate. The res of the cintiq is too small, also it's all a bit too non comfortable. The 2.5 inches around screen borders being a non accurate area is no good in that small res, the dual screen mode at 1280x800 wont work well in many cards, just single screen mode, the calibration...you'd better draw always in same position, or you'll see not the cursor when you want it, and finally, like in all tablets, there's a lag between your stroke, the speed and response you get with traditional pencil and paper, and what actually happens in the cintiq. Like in all tablets, wacom or not, but is much more noticeable in a direct to screen device.
In the cintiq's positive side (less points but strong ones, depends on your needs) some people do need to paint like in traditional way, direct to screen (I'm a traditional painter, but I don't mind), it is actually a really good tft screen, could have kept it (sold it) just due to the sRGB good profile, the great colors needing little adjustment from default, rare thing in a tft screen. I was pretty able to do professional work with it after some config, and I was happy with it. Just does not worth the price, and even if it were cheap, I'd go anytime after all my tests and usage, towards an L or XL intuos4 model.
The cintiq 21" , that's gotta be different(have not tested). Enough resolution to be accurate for line art, the 2.5 inches of non accuracy in the borders matter less in such a big screen... I bet is a good purchase, but again, too much money for my pocket (2000$ !) to get maybe less accuracy than with the XL (790$, even cheaper than the 1k of the 12")
About the scratches and dust problem for cintiqs and intuos that floats everywhere on the internet... I have never ever scratched my tablets. I have seen work buddies ruin a pair of those, but doing really a savage usage of it (including football activities).The dust...never a prob, although I cover the cintiq and xl, when not using. But I'm careful with stuff, generally, every person is a different world, as with ergonomics. If you press quite when you draw no matter what you are using, then you can have the problem. Some people don't even know how strongly they press against the surface..
Oh, and..it shows my age, but I have owned a Kurta tablet...The first tablet for artists, or one of them. Also Bestbuy and Genius. I'm an old artist.
Hoping this 2c opinion (from personal experience) might help someone.
"Disclaimer" : I have written before about Wacoms, slightly different opinions. I just change my views as I get to get more products, certain tests, usages, etc. Always my latest post on the matter will be more "accurate".