It depends on what and where you want to study. If you can only buy one sensor and do most of your studies indoors, you probably want to buy the UVA Sensor (UVA-BTA). This is because you will find more UVA-emitting sources available than UVB-emitting sources. If you can be outdoors and want look at the more sunburn-inducing radiation, choose the UVB Sensor (UVB-BTA). Note that UVA is now thought to be as much of a cancer risk as UVB.

Some of the most interesting experiments, however, look at both UVA radiation (longer-wavelength UV light) and UVB radiation (shorter-wavelength UV light) in sunlight. This means using both UVA and UVB Sensors in order to do a comparison.