B.V.
Hi, Much depends on the educational experience you want for your child. Arlington and Belmont are towns. Somerville and Cambridge are cities. Travel is much easier in Cambridge and Somerville because of the number of subway stops and bus routes. If you won't have a car you should keep that in mind. You don't want to feel stranded. BTW, all 4 locations are very close to one another just to give you a distance perspective. I live in Cambridge near Harvard and I can walk, take the subway or a bus to Somerville, Arlington or Belmont, each is 1-2 miles away.
As far as education goes, Arlington and Belmont are not really considered diverse meaning they don't have much of an Asian, Black or Hispanic population. Cambridge and Somerville do. In Cambridge you have choice in the school system so you can look at the various elementary schools and decide which one you want your child to attend. There are different strengths and teaching styles. e.g. there is a school that has Spanish immersion (they children are taught one week in English and one week in Spanish), another has daily Mandarin instruction starting in Kindergarten, another has music 4 days a week. I am not sure if choice is the method in Somerville. In Arlington and Belmont you just go to the school that is closest to your home. All schools in Cambridge offer full day kindergarten although you can choose to pick up your child whenever you want. If you want to ask any questions or send me the age(s) of your child(ren) I can possible help with more information. B.