These are 2 tablets with very similar specs and nearly similar size, only you can answer to yourself which size is more suitable for you. Its best that you go down and have a feel of the tablets for yourself.
I personally went through my course with just a EEE PC 10.1 Netbook, i never felt it to be small or inadequate for my projects, lectures etc. The only thing I wished was that it had a stylus input.
Between the 2 tablets though, I would go for the Thinkpad Tablet 2. During my time with it, I find the stylus to work better than the Ativ and the TP2 feels more balanced on the hand than the Ativ. Portability wise, they are nearly identical, but again I prefer the feel of the TP2.
But whether its the TP2 or Ativ, 1k+ for a netbook with stylus? Hard to swallow =(