Both are very good. I do not think that I would particularly prefer one to the other other than based on cost, and possibly travel convenience or just personal preference.
One daughter started off with a major in environmental sciences, with particular interest in biology. She switched to biology, and then switched to animal sciences. This sort of “adjustment” between somewhat similar majors is quite common (and might be relatively easier for students who switch early enough to graduate on time and on budget).
Data science as I understand it is sort of somewhere between computer science and mathematics (and statistics), which are areas where I spent much of my education and career and for which both U.Mass Amherst and UMD are quite good.
How you would pay for the cost difference is important, although I think that your son could do quite well at U.Mass. To me this also seems like an area where a master’s degree (whether right after a bachelor’s or a bit later after some work experience) is possible, so the cost savings of staying in-state might be helpful if it allows you the option of helping to fund a master’s degree.