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1. Range: There's a big difference in range between the cheapest helmet intercoms and more expensive models. Affordable intercoms only allow communication with another unit if your buddy is riding pillion. At the other end of the spectrum is a range of several kilometers, but such technology comes at a price, and thus, a long range increases the cost of the helmet intercom
2. Connectivity and Group Calls: Similar to the above, a cheaper unit usually offers a Bluetooth connection to, for example, your phone and possibly to one other helmet intercom. However, if you invest more money, you'll get better technology that enables connection to multiple helmet intercoms and multiple Bluetooth devices simultaneously
3. Bluetooth, Intercom, and Mesh: By default, all helmet intercoms we sell support Bluetooth connectivity to at least one other helmet intercom. Intercom is a radio system that allows connection to one or more units, and Mesh is also a network that enables phone connections between large groups. So, the better helmet intercom you get, the better communication channels it includes and the larger group calls it enables.
4. Helmet Compatibility: Almost all modern helmets allow for the installation of a helmet intercom. For example, Cardo and Sena intercoms are "universal," meaning they fit almost all helmet models that simply have space for speakers and other components. You can easily check your helmet to see if there are, for example, recesses for the speakers where they fit. Model-specific helmet intercoms (Shoei, HJC, and Schuberth) are designed according to the helmet and integrate fully with it. They are usually the easiest and most stylish way to attach an intercom to your helmet. 

Ask for help if you don't know which helmet intercom to choose!