I just recently changed my Clarisonic Mia brush head for the first time. I started off with the Acne brush head, and now am using the Sensitive brush head. Clarisonic suggests to change the brushes every 3 months. The sensitive brush head is actually the one that came in my Clarisonic set, and I bought the acne one separately, but now I'm trying the sensitive one.
Left is the acne one, and on the right is the sensitive one. A key difference between the two is that the acne brush's bristles are more compact together yet separate, and the sensitive one has fuller bristles, thus making it more dense. (note the difference below). It is still to early to tell which one I like better, but as of right now, I think I'm leaning more towards the sensitive one.