As with the previous post, this seems to be missing the point that grad school isn't really about the same thing as YCombinator. Grad school is about advancing the state of scientific knowledge, which might result in useful products or services, but not necessarily, and often not in the short-term. Startups are about producing useful products or services, getting them to market, and monetizing them, which might result in scientific advances, but not necessarily, and often you actually hope not to need any (needing to solve an open scientific question in the critical path to getting a product out the door is a huge risk).
I think if you are asking yourself and others this question then you aren't ready for either. My advice for anyone who is confused about this is to get a job in your field of study first, then after a time you will know what the answer is.
You'll probably get a biased view on a Ycombinator forum, but I think in essence the decision is pretty easy:<p>If you like to go deep into problems, conducting basic science, and attempting to solve fundamentally difficult problems that may ultimately be intractable, and if deadlines stress you out and you feel like you don't get a chance to do anything "right" and if you have the ability to focus on the same thing for months or years at time, then grad school will probably suit you well.<p>On the other hand, if you more interested in getting tangible results fast, and getting other people interested in what your doing, and providing a product that regular people find valuable, and you're willing to change direction a lot and throw out good work for no other reason than that market isn't biting, then doing a startup will probably suit you well.
If you successfully exit a startup, you will likely be wealthy: i.e., you will never be forced to take a job in order to eat. If you successfully finish grad school, you will probably be no closer to becoming wealthy than you are now. So if you're ultimate goal is to become wealthy, start now.<p>What keeps most people from becoming wealthy is not a lack of grad-school-type knowledge.
grad school is a 5+ year commitment. Launching a startup probably takes less time. Also, grad. school is not about entrepreneurship. Perhaps there are transferable skills, but research has different goals.