I don't really think its necessary to read either BRS or Costanzo this summer, as phys is really most helpful to review when you can integrate it with your path subjects during your second year. Also, you should enjoy the hell out of your last summer (just my opinion).
As for which book to read and when, in my experience I found it much more useful to read the detailed Costanzo during MS2 and the more concise BRS phys for boards. I believe that BRS phys is enough for boards (it was in my case) and that not reading Costanzo for board study would not hurt your score. However, I think what a soon-to-be MS2 should be focusing on now is not board study but making sure that you understand the phys as you learn the path this upcoming year, and whether you use Costanzo or BRS to accomplish that depends on how good your phys foundation is and how much time you want to devote to it.