Wife and I had Scala G4 for a few years but got rid and changed to Sena SMH10 a couple of months ago. Problem with the G4 was that the bases kept breaking - one or both speakers would just stop working. Between the pair of us we went through 5 bases. This included the upgraded G9 base which also suffered the same. Couple of friends had same issue. We also had frequent issues where they'd crash or lose connection with the Zumo.
Sena has been fantastic. Speakers aren't quite as good as the G4/G9 but I use plugs with built in speakers which work perfectly. This is a standard option for the SMH10 but not an option on the Cardo units. Apparently the newer Sena (20s) has far better speakers. If you need an FM radio then the 20s has one, but the SMH10 doesn't.
We've found no drop-out/crashing issues with the SMH10. They are also very easy to operate as there are only 2 buttons. Since we have the phones connected through Zumos we don't even need the second button (phone) and can do everything from the jog wheel.
Spent the last 10 days around France, Belgium, Switzerland and Germany and had no issues at all. Also of use was the universal pairing, which means a Sena can talk to a Scala and other makes, as a mate still has a G4. This worked flawlessly for us. The only side effect is that the G4/G9 users can't use theirs with their phone while universal intercom being used but this wasn't a problem for him.
We were getting 2 days out of the units without charging and there was a lot of sat-nav and intercom in use. The SMH10 can also do music sharing, so you can have both units listening to the same music source if required. We only tried this once, purely by accident, so can't comment on how good it is.
Personally I'd go with the 20s if funds allow, but the SMH10 is a very good unit and I can't speak highly enough of it. I'd never go back to Scala based on the experience of mine, the wife's and our friend's. They were good when working but too many reliability issues.
Scala don't seem to do many software updates whereas Sena seem to release them as soon as someone identifies an issue. They're also adding new features all the time like the recent sidetone addition (you can hear yourself when talking if you enable it so you can gauge how loud you need to talk) and the universal pairing function.
Dae.