I think the screens would stay the same, 21.5 is easy for production. I was on the same fence two years ago and pushed out the extra for a 27inch and i love it. Blows away my old G5 20 inch imac lol.
I know, I know
That's why I mentioned my dad's opninion...
So, I think it will be the 27 one for me too. It is nothing more then some "dreaming" or so, nothing more, because I know Apple won't do any changes. Need to protect myself from making any illusion
The USB3, instead, is indeed serious for me.