Looks like Pepper spray for personal defense is finally legal in Massachusetts without a gun permit. It had been classified as ammunition and the penalties for carrying it without a gun permit were draconian.
Here is the article.
http://www.recorder.com/home/1358841...without-an-fid
If one makes the personal decision to carry pepper spray, I think it only makes sense to carry the most powerful concentration available and legal in the areas you intend to carry-- in MA that can be the same strength that police carry. The friendly folks at your neighborhood gun shop can help you figure that out or research on-line.
Other consideration include whether the spray is a stream (goes farther) or a cone (easier to hit the assailant) and whether or not the spray has a UV or colored die marker. On that point, I am thinking better not to have if you are not a LEO. You will also need to figure out the best size-- from something that could go on a key chain on up.
Here is a link to one well-regarded manufacturer:
http://www.foxlabs.com/products/pepper-spray.html?