I know you put Dave's name in the title, but thought you might like other opinions as well. Hopefully, Dave will mention some things I forgot.
I also ride mostly solo, and recently upgraded from the SMH10 to the 10S. Your plan to keep the SMH10 for the occasional duel ride will work well. I gave mine to my grandson and when he and I ride together, the two headsets fine together.
The improvement I appreciate most in the 10S is the music fidelity. It's still not as good as a quality set of stereo ear buds, but the music is much more crisp and clear than the SMH10 was - and there's plenty of volume. Other improvements I like is the new "selective pairing" mode, which allows more flexibility when pairing to several different devices. There is a related feature that I think is called "audio override" which allows me to seamlessly switch between listening to music from my iPod via the SM10 BT dongle, or listening to Pandora on my phone. I no longer have to get off the Spyder to turn off the SM10. I can now just control the switching by turning on the A2DP protocol for the 10S in the phone's BT controls, allowing music from the phone to "override" the SM10 dongle. (Hands free phone connection is always on, regardless of the music source.) Lastly, the Bluetooth 4.1 upgrade outshines the BT version 3.0 on the SMH10. It is both more dependable, and also more power efficient, giving the S10 a long battery life. The 10S is also smaller, sleeker, and the jog dial is a little easier to operate. All in all, a very nice upgrade from the SMH10. :thumbup:
The more expensive 20S has all of these features, plus enhanced group riding capability (more group connections and longer range).
In case you do go for the 10S upgrade, I've attached some paring hints that I wrote during my initial set-up. You may find them useful, since you and I have a similar "riding style". (There are, of course, several different ways to pair various devices, depending on your individual needs.)
