I have a 20 and 20S, and an ancient SMH-10 somewhere. Sena is one of those names like Garmin. Some like them, some think they're evil. My history with them has been mixed. That said...
In the Sena line, I have yet to hear of anyone who's happy they have a 30K. It seems to be a bad implementation of a network system coming into use. Particularly if you ride alone or with only a couple of other folks, there's zero need for the 30.
The 20S is a slightly tuned up 20. There's no need for a pop-up FM antenna, and supposedly the Bluetooth (BT) antenna is somehow better. Having both, staying in contact with the one person I ride with (another bike), there's no difference. Otherwise, they're so similar the firmware updates have been the same for both units. Having a 20 doesn't mean giving up anything that matters.
As to living with the thing, it's pretty much transparent. It works, I don't need to fiddle with it, I just turn it on and go. But... pairing, even with another Sena unit, is a PITA. Maybe it's a user thing, maybe it's a Sena thing. It's been a non-stop PITA. Connecting with phones, GPS' is no problem. Follow the steps in the book, and life is good. One of the details that is "wouldn't think of it if you didn't have it" is the ambient mic. That is, enable the mic and hear the outside world with needing to take off your helmet, or sound like a fool from trying to figure out what someone's saying. There is a small tip with the mic. It's next to the charging port. Close the port's cover, and the mic can't hear anything. Leaving the cover open isn't a biggie except in real rain.
Rain... hoo! I've worn my 20 in serious, frog-choker rain on I-95 with no problem from the 20. Some people report they drown in a heavy dew. I have no good answer for this except I think the 20 has some attempt at seals. NTL Sena doesn't say the 20,S or 30K are weatherproof. YMMV
I bought the 20 because of a couple of things I didn't like about the SMH-10 (very scary, too ready to let go mount, for example). The 20 mount is solid and the 20 isn't going anywhere except in a dump. (the SMH-10 cannot work on a 20 mount or vice versa). I bought the 20S out of "It's later, therefore it's better" non-thinking. Silly me.
Now, all of that said, Cardos do well in reviews. I haven't owned or used one. My opinion on that would be worthless.
, the Cardo battery isn't user replaceable. The Sena battery is, for sure not user replaceable but... There are a few YT vids on how to replace the battery. It takes care, patience, and a little willingness to make something fit with some "customization". It's that or pay Sena for a battery change. I've done the job without too much trauma. Look at the videos and decide.
Otherwise, I've needed no support on the 20. I have reported issues to Sena with zero feedback. That's annoying but doesn't keep me grounded. Meh.