Location: I really don't see the NYC advantage. It's even MORE expensive than SV, there are fewer angels and VCs, and the culture worships investment banking instead of startups. I would say NYC is probably the second best place to start a startup after SV, but it's a distant second.
Mentorship and events: This does seem to accurately describe a difference between the two programs. In YC you'd never[1] spend an entire month (let alone the first month) in meetings. And YC is mostly about advice from YC rather than from a larger group of mentors. [2]
Size of the program: YC is much bigger than TechStars. Of course, YC can be bigger because it gets so many more applicants. While you may share access to the YC partners with more startups, they commit enough time to make sure everyone has all the access they need.
Demo Day: If you've ever been to a YC demo day, you know it's incredibly intense. Three days of presentations to many of the top investors in the world, packing the house. The presentations may be shorter, but it doesn't seem to hurt anyone's ability to raise money.[3]
[1] There are probably exceptions to this, for people who somehow already have products coming into YC, but offhand I can't think of one.
[2] Of course YC can introduce you to pretty much anyone at this point due to the size of the network, but I don't think that's really the central point of YC.
[3] Side note: I find the complaint that the pitches are "informal" a little weird, since I think one of the things that makes startups so dangerous is exactly that they are allowed to skip the formalities and cut to the chase.