My Sena 20S BT headset is rated for a range of 1.7 miles. With two 20S units, I find the range to exceed 1.5 miles in the flats. As previously stated, the limitation is keeping multiple headsets connected, requiring a lot of work each time a headset is turned off and back on. Between bike and passenger or between two bikes, BT is a good option.
When I installed my CB radio, I had a lot of difficulty finding any radio traffic for testing. I thought I had a bad unit until I asked a friend with a CB to transmit. I heard him loud and clear, even from miles away. I was told that CB has mostly fallen by the wayside. I don't know what truckers are using these days in lieu of CB.
FRS is inexpensive with Bluetooth-capable FRS/GMRS handheld units running less than $100 (and I see units available for less than $50). They can get their power from a USB port or simply run off the internal battery. You can use a BT headset to connect, or wire the unit to your bike's audio system, which allows you to use a cabled headset. The units easily fit in the glovebox or trunk. If you frequently ride with the same people, a cheap FRS radio becomes a good option. I used to take the family camping. I would ride my bike and the family would ride in the cage. We used FRS handhelds and were able to stay in touch over distances in excess of 5 miles. If you look around, you can find inexpensive BT-capable FRS units. That may be enough to sway people you ride with over to FRS.
Although I dislike group riding, I wanted to be able to connect with anyone I was riding along with. I opted for the BikeMP3 Evolution CB radio with the FRS cable option. I connected a Motorola FRS/GMRS handheld radio (top-of-the-line, less than $100) to the Evolution. I use a Sena SM10 to connect to the bike's audio system. The limitation of the SM10 is that it cannot be used to talk, only listen. Until I replace it with the upcoming Freewire, I connect my Sena 20S headset directly to the Motorola radio. I could have left the CB unit out the way it is currently wired. Downside is that I have to either use VOX (Voice Operated Switch), which transmits every cough, sniffle, mutter and wind noise, or press the Talk switch on the headset each time I want to transmit. Changing channels is manual, but normally you would select a channel before you start your ride. Once I install the Freewire, I will be able to use the bike's PTT (Push To Talk) switch and have full access to the CB radio including the FRS/GMRS radio.
There are a lot of options out there. Some work well and others have problems. The limiting factors are what the people you ride with are using and how much you are willing to spend. If you opt for CB, I recommend BikeMP3's Evolution. It is far less expensive than the OEM radio even if you add the FRS/GMRS option. In fact, the Evolution with an additional FRS/GPRS radio is still far less expensive then the OEM CB radio. Pete provides excellent technical support as many on this forum will attest, and installation is easy. The web site is bikemp3.com.
You should now have enough information from this thread to make the choice that best suits you. Good luck.