Accuracy is likely the most important factor when making a breathalyzer purchase, so you should consider the Sensor Type – semiconductor versus fuel cell. See this question for information about the differences between these sensor types. Essentially, both options will provide accurate readings and the degree of accuracy will increase with the price.
FDA Clearance is another important requirement to consider when choosing a breathalyzer. Every personal breathalyzer sold through BACtrack.com meets the standards of an FDA 510(k) cleared device, which means the product has been deemed "safe and effective" for consumer use.
Breath Sample Collection is an essential factor, and critical to an accurate breathalyzer reading. While the use of mouthpieces is one element that aids in obtaining breath samples that exclude external air, other aids to accuracy are found in the actual product design. For instance, the miniature air pump featured in BACtrack Mobile, BACtrack S80 Pro BACtrack S75 Pro, and BACtrack Element breathalyzers allows for the highest level of precision in collecting a breath sample. During use, the user can actually feel the "click" of the pump collecting the breath sample.
Lastly, you should look at the brand of breathalyzer you are purchasing. The manufacturer should be reputable, in the business of selling breathalyzers, specifically, for an extended period of time and have the endorsement of professionals and press as a result of providing a quality and accurate product.