Which school should I attend?
Andover.
Deerfield. There are several kids we know who had this fortunate choice. Every time they head up to Andover - they are happy they chose Deerfield. Different types of communities and different size schools.
OP: Not enough information shared in order to offer a reasonable suggestion.
@OneMoreToGo2021 - by now you may have already made your decision. But to agree with @Publisher - you may want to provide more information. What I meant about different communities and different sizes needs to be considered by what you are seeking.
Has the OP ever visited the schools?
I know that several families have not yet had the opportunity to visit schools in person and that they were waiting for Revisit to take the trip. It must be very difficult to try to decide between schools that you have never seen in person.
Andover is a larger school and has more students, more courses, more choices of residence life, etc. You may also be interested in a particular sport, performing art, computer program, area of study that would be better served at one particular school over the other. Also location-setting considerations.
Very different communities, location, setting, and experiences. So, for the OP, I would recommend making a list of what your goals, expectations and visions are for boarding school - as the type of community experience you are seeking.
A significant difference between Andover & Deerfield Academy is access to Boston & frequency of students heading into Boston.
What were your reasons for applying to both? What attracted you to each? Internet strangers can post perspectives and opinions that may be entirely different than the perspectives and opinions that you would form from attending either school. You will have to make this decision on your own with your family as no one here knows what would be best for you. At the very least, you need to provide a LOT more information than simply “Which school should I attend?” for any response to be at all meaningful.