Hello Lindy,
although Sal and Boavista are not overcrowded like beaches in Italy or in other European countries, the lower frequented islands would be the better choice for you.
As Daniel wrote, you can combine different islands and your holidays become twice interesting. 😀
But be clear on the fact, that these islands are not as developed as Sal or Boavista!
But this might be also the charm of your journey. You will discover the real Cape Verde instead of wasting your time in a sterile all inclusiv area, where no nativness can be found.
Please notice, that Portugese is a necessity (even a few words), because English is nearly nowhere spoken, with the exeption of the airports! ❗
I wish you pleasant holidays on the islands. 😛
Ariminum