The D branch is easily the best of the Green Line branches though as it is a dedicated track and doesn't have to stop at traffic lights or due to traffic. It is also the easiest branch to access Longwood from, it's only a 20-25 minute ride on the D branch from Newton Centre to the Longwood stop.

I agree commuting around the Boston area is generally not the best, but that particular commute is honestly quite easy. But I can see what you mean that if they can afford Newton Centre why not just go to Brookline anyways and be even closer. It depends what OP wants though, Newton is a little more suburban than Brookline is overall, though how dense each is really depends on where you are in each town.