Re: New C3 Pro and Sena 20S
The 10U and 20S both have bluetooth 4.1.
Some of the advantages over the SMH10 include much clearer intercom, much better noise cancellation when you are talking, being able to connect to multiple devices at once, and also audio multitasking. This means that you can have the music on (radio or bluetooth music) in the background while you are having an intercom conversation. You can also get GPS directions while talking and still listening to music. It's pretty cool.
There are a couple of advantages to the 10U, as stated above. The form factor is much sleeker, but for full funcitonality, you need a remote control that replaces the multi-wheel thingy on the 20S. May or may not matter to you. The 10U cannot be charged while you're using it. Again, may or may not matter to you, depending on how long you ride and if you forget and leave the intercom on, draining the battery.
I needed to use the sticky mount for my 20S with C3 Pro, before making a modification to my helmet that many would not want to make, in order to use the clamp mount. Also, the 20S has to be mounted a little further back on the helmet than I'd prefer, due to the sunshade mechanism.
For me, needing the remote (Sena now makes a smaller remote, but I don't have room on my handlebars for it.) and not being able to charge while using made me choose the 20S. YMMV.