I got 22HD and 24HDT, it's lovely to draw on, very fast to reposition your hand to the exact spot you want to tweak, and the glass surface doesn't wear down your nibs so much. You don't draw any better on a big screen, but you save some time (zoom in -> zoom out) when you review your images. A big screen is nice when retouching details.
If you already have a second monitor to pair with the 13HD, I'd figure it could be a decent combination to have the drawing area dragged to the 13HD (for example photoshop, you can move the drawing area "out" of the program and make it fullscreen on other screen. Then you'd have all that fancy fullhd 13" space only for the art without all the other stuff that you can access with keybinds and with mouse on the other monitor.
I can weigh in an say I have a 22"HD Cintiq. I do love it, however, it is huge, heavy and bulky. It takes up a lot of real estate on my desk. The 13" is really tempting me. Under 5 pounds, very thin, and one cable (although it is placed in a weird location). I am very much considering selling my 22 and getting a 13.
So you see the conveniance of weight and the more compact size more valuable than the area of work? I've never head that response. But I can understand. The 22ux at uni is heavy to move. I don't know if you found this but if your not looking down the pen on the tablet, the depth of where the display is and the contact of the screen creates an odd sensation as the line doesnt appear directly under the pen. Maybe its just Im use to not looking at what I draw with the bamboo Thanks for chipping in. I'm hoping the funds crawl across the line to grab one. The bamboo is asking for retirement
There are ups and downs. Don't get me wrong, the 22 is awesome. I do however find that I only use a certain section of the full monitor anyway, so I don't think having a 13 will hinder that. Rarely do I use the entire screen to draw. I bet if I could see some sort of 'use map' there would be a general area in one section, most likely because that's the area by arm/hand rests while drawing. I do think I will pick up a 13" anyway and give it a shot. I do like that's it's 1920x1080, which is also a step up from the 12UX. Let me know how it turns out if you get one! Good luck!
While I haven't used the 12ux...I have used plenty of Tablet PCs with the same relative size and res.
I know some Artists have complained about feeling cramped...but I make do just fine with the 11.6 inch screen in my S7S. I also have the 21ux Cintiq...while I love its big screen very dearly...I can alternate back and forth from 11.6 to 21 inches with no prob for my general work flow.
I've also had plenty of the more traditional pen tablets, Bamboos, Intuos 4, etc...while I got very proficient at using them....I still prefer drawing on screen. It just works works better for me.
I would recommend going with the 13HD.....but more so if anything for the benefit of having a Dual Screen set up. Until I got my 21ux...I never realized just how useful having dual screens can be....especially for art purposes, since you can throw reference photos or tutorials on your old monitor while you work on the Cintiq.