Another area of discussion is the cost of running a startup. Most tend to believe that it's more expensive to operate a startup in NYC than it is in Silicon Valley. As a former accountant, I've spent some time crunching the numbers to see if there is, in fact, a difference. I'd suggest that overall the total costs are nearly identical. Office space is more expensive in NYC but other costs are higher in the valley. For example, transportation costs are higher on the West Coast. In NYC, you can get by without a car by using the very inexpensive public transportation which blankets the entire city. Out West you need a car, as public transportation overall is spotty.
Lastly, the type of startup can help determine which coast will provide a better chance at success. What I've seen is that if you are trying to build a social network or a service that's looking for a massive user base and low per-user revenue, the valley is the place to be. The attitude in the valley is much more accepting of a service that might take a while to realize its financial potential. In NYC, you better have your business plan worked out the moment the "launch" button is pressed. Many NYC startups also focus on services that will have fewer users but a higher revenue per-user. Also, the word on the street is that if you are building a media company or plan to work with media companies, NYC is the better choice.
In summary, I'd suggest that all entrepreneurs research both coasts (and everywhere in between) to determine where is the best place for your specific startup. Each location offers so many wonderful benefits that there is no single model that can be easily applied to determine the best location.