12.9 definitely.
I am officially using it as a laptop replacement. I am a graphic artist. I also do hard core video editing. But I also use it as just a really portable device which has replaced everything I did on the MBP 13". Since my MBP was a mid 2012 model, the new IPP actually outperforms it processor wise anyway.
As for size.... I am 5'2" tall with small hands. I am female. I have absolutely NOT ONE SINGLE issue with the weight or size of the 12.9" IPP. I take it to bed with me, I hold it on the sofa to surf while watching TV, I take it to Starbucks. I have a stand I use for hardcore typing, along with the BT Apple Magic keyboard. Then it feels like a mini iMac. I can have two apps side by side and they are the FULL apps, and not the phone apps. On the 10" one, the side by side still punts the apps into phone mode. The larger screen also allows for 16:9 movies etc to still be large enough to look awesome. Also, the speakers on the 12.9" are unmatched by the 9.7 or 10.5 versions.
I really don't understand how people (mostly big burly guys who have much larger hands than I do) somehow think the 12.9" IPP is so huge to handle. LOL. In reality we are talking about 2" diagonal difference, that's it!
I know if we were speaking about anything else, 2" more would not be a problem to handle would it?? Haha