My answer is "it depends on the kind of study you plan to run".
There are several firms in the market selling eye tracking devices based on infrared (for serious research I don't trust yet in other technologies based on recording the users' ayes with a webcam). As an infrared-based technology I know Tobii. I have a Tobii T1750, a second-hand one in fact from 2007, that it is not anymore in the market, so I cannot be sure about the newer models to recommend, but here you are some models and the uses that I (and Tobii) recommend for each one:
- Tobii T60 hz and T120. Useful for research that aim to study the users' behavior on websites, images, and anything that can be showed on the screen. This device brings the infrared on the screen. Useful for usability studies
- Tobii X60 and X120. Useful for research on screen (external screen) or other devices as mobile phones or tablets if you incorporates a special device for them. It is a great option because is lighter and flexible. If I could buy another one now, this would be my first option.
- Tobii T60 XL. Similar to T60 but wider. Useful for studies that need a big screen. I don't see the point for usability studies.
- Tobii Glasses. Necessary if the research is about physical objects (supermarket products, museums, etc.). I used it in a study on Connected TV and video games. It works properly but the resolution is lower than in screen-based eye trackers, and the complexity for the analysis is bigger due to the kind of information: video. It is not convenient for web studies.