I use a Cintiq 21UX and love it. You need to get used to having your hand “in the way”, I learned drawing the old school way with paper and pencil so it wasn’t a problem for me. I like the feel that I am in contact with what I am working on giving me a sense of being more connected with my work.
All that said the 21UX has not quite twice the real estate of the 12WX. My hand covers maybe 1/8th of the work space when I am working. With the 12WX a hand might cover as much as 1/4 of the working space (not a deal breaker but something to think about).
The 21UX is 22 Lbs and big, so it’s not something you want to pick up and hold in your lap (although it can be done). The 12WX is less than 5 Lbs so it’s feasible to hold it nearly anyway you like (what’s comfortable to you).
I also have a large Intuos 3 (12 inch tablet) that I use on the same machine and I love that too. So, bottom line I would say it boils down to personal preference. If you are working professionally and can justify the expense you might consider the Cintiq; nevertheless, a large tablet is a pefectly valid choice and, depending on your work habits and financial considerations, it may be a better option.