Both
I fitted a pair of SMH 10 a few years ago, and have been very happy with them.
A few months ago my helmet dropped on the floor landing right on the unit. it bent all of the pins on the bottom row of the main unit, and just a word of warning, don't try and bend them, they all snapped off.
No chance off buying just a main unit in Australia, they are permanently out of stock.
My partner ordered a new set for Christmas but she ordered the SMH 5.
I use a Nolan N102 helmet and the SMH 10 works ok, but I was never happy with the amount off wind noise from the back seat, which was in a normal full face helmet.
I fitted the SMH 5 unit into her helmet, and the wind noise is all but gone due to the smaller stick on Microphone.
Both very easy to use. The SMH5 is a smaller lighter unit, but seems to work better.
All in all, if anyone is thinking of fitting a pair off intercom's I would recommend either of these units.
As a mate said, " you spent $300 so you could talk to your partner on the back? I would sooner have spent $300 not to"
Each to their own.