I also have the 12" and I like mine because of the size, it doesn't bother me like it does to some but I always have carried a drawing pad about the same size with me so it's just a personal feel.
As of today I've had mine for about a year and if you can afford it, go for the larger Cintiq if you can. I don't have issues as some have with the 12" but I think it's all about the times they were manufactured, it happens everywhere that one has a bad week(s) with any production not just Wacoms in general.
The better thing about the larger units, the stand is really nice and stays where you need it to stay. The 12" has a floating bar if you will that doesn't lock in certain positions so if you push with your forearm a bit on the side it will slide down a bit. Don't be fooled by the claim of small space with the 12" as it still has full sized cables and a base unit (Wacom seems to not discuss this point).
I'd say if you draw with large strokes then you will want the larger unit, small to medium strokes, the small 12" would be okay but the large gives a more flexible field of drawing, painting and so on. With the 12" I always have my pallets and everything I can over on my 17" MBP's screen, only the canvas stays on the Cintiq. If I had the 21" I would have a few things on the same screen but not many. Now for anything 3D wise, go for the bigger unit. Trying to do things with the 12" in 3D makes life a bit iffy, still doable but you have to make it work, not the other way around
I got mine for a deal because Amazon had a 6 hour special at the time I was looking and it was only on the 12" so if by chance I didn't like it, not a big deal to send it back or sell it for what I paid and break even. Like I said, I like mine but the 21" would be nicer yet I still get more done with the 12" than with my 9x12 Wacom tablet because drawing on the screen is just nicer and faster for me in the long run.
Good luck and keep an eye out for daily deals. There use to be a site that would notify you if a product you wanted dropped in price because of a sale however I don't remember what it was. I think it was posted somewhere here in MR but mroogle doesn't work for me anymore so the search feature isn't finding it, sorry
Oh last, check out some drawing stuff on youtube, some folks where a half glove type of sorts so the screen stays clean as your hand rubs across it all the time even if you don't mean for it to. Great idea to preserve the screen in the long run and your hand just glides across like on ice