Since they started paying for teaching (a change in effect for Fall's incoming graduate class), it seems like NYU is going to outstrip Columbia's funding. Both schools offer fellowships of about 23,000, and though Columbia will fund a 6th year, NYU allows you to "bank" the earnings from one year of teaching to fund a sixth year. (But you don't get the money back if you finish your dissertation in the fifth year and are ready to go on the job market then). However, the NYU teaching money will come only in the later years, so the first two years of study will be at the same 23,000 as Columbia. I think the total teaching load is four semesters spread out over the 3rd to 5th years. The 22k is the total amount possible to earn for four semesters taught over those 3 years, so it seems doubtful that there would be one year approaching total funding of 50k. Some years teaching it might only raise your funds by $5,500, but it is still a pretty good deal. And it's nice to be paid for teaching rather than simply be doing required "service."
I hadn't heard about the possibility of summer funding. The faculty and students I spoke with left me with the impression that summers would likely remain unfunded. But getting by on 23k in NYC must be possible, because so many students do it - right?