Its beyond discussion that as of today, the two top motorcycle communication unit brands are Cardo and Sena. There are several posts that explain the technical differences between Sena and Cardo so I wont be spending time on that. Its a never ending debate between Cardo and Sena! I wont tell you to use Cardo because I use it and like it and also that I have been equally happy with Sena in the past. When you compare the two, its important to make sure they are comparing two similar models. Each of the brands have so many different models that its not fair to compare a Cardo without DMC to a Sena unit that uses Mesh intercom.
Leaving their minor differences aside, the most important factor to consider is if you will be using the communication system to connect with 3 or more riders. Once you have that answer, the choice is pretty straightforward
Group Riding (3 or more) : Even though Cardo & Sena connect to each other via Bluetooth they wont connect to each other using their primary technology (DMC for Cardo vs Mesh Intercom for Sena). If you ever connected a combination of Sena and Cardo with more than 2 units, you will know that its a nightmare right from getting them to communicate and holding on to the connection. You will lose all advanced features offered by both these when you use Bluetooth to connect.
Short answer - Buy the communication unit that most of your group riders use and you will have no complaints
Non Group Riding (Music & Phone Calls) : If you are not into group riding, pick any headset (even a non Sena/Cardo) and you cant go wrong. They all go a good job with plain old bluetooth for phone calls and music
Good luck and safe riding!