The US is big in every sense of the word, cost of living varies hugely in different areas. There are places where $60k is more than enough to live nicely, and there are places where $60k would mean sharing an apartment, and being very careful about spending your money.

One thing to keep in mind is that you might $100k+ in student loan debt by the time you actually get your phd. So for that reason you go for the higher paying job.