I use the Sena SMH-10r. It's the same guys as the SMH-10, but it's WAY smaller. The battery is relocated away from the main unit so you can mount it on the back. I had enough room to mount it between the foam and outer shell of my Schuberth S2.
The Sena options are cheaper, but more people have the Scala systems. If you expect to ride in a group, most likely your group will have the scala G4 or G9. They also have greater range and number of people in the net at one time.
Everyone I've talked to that has used both scalar and sena have liked the sena better. I chose the sena based on features, price and buddies that have them even though the schuberth S2 is pre wired for a schuberth branded scala G4.