In terms of education, network, career opportunities, each school would be superb. If someone had a strong preference for the Berkshire beauty of Williams or the consortium opportunities at Amherst, that could make a difference.
Otherwise, I agree with @shuttlebus that athletics could be the tipping point. We were always told as my kid went through D3 recruiting – think about what if you had a career ending injury the first day of preseason, would you still be happy to be there? If the answer to that at both schools is “yes,” then, it comes down to team culture. How do you feel you fit, both athletically and personally, with the team. Have you asked about whether recruits are cut, and how many students with your level of admissions support and similar stats were deferred or rejected in ED round?