A lot of the purchase decision will require information to your particular environment and cleaning needs. If you are looking for simplicity and turn-key function and low cost look for the 6XX models. There are even WiFi models in the 6XX, 7XX, and 8XX models. The 9XX models are the first models that utilize VSLAM hardware for room mapping using digital cameras instead of LIDAR units. These 900 models provide a digital image of the cleaning area completed via an Android or iOS device that is also used to control the Roomba as well as schedule cleaning times.
If you are a person that enjoys playing with your Roomba, they are all equipped with Open interface software that lets you communicate with the internal motors and sensors to do different tasks. That same interface also has the ability to provide all the diagnostic information of your Roomba. If you like to tinker, all of the iRobot Models have various mods that can be performed on them to improve longevity and reduce some maintenance activity.
There are some sellers on
Amazon and
ebay that offer earlier models like the 5XX and even the 110 Dirt Dog can be found as well as the 4XX and 4XXX models. These previously used models will also be lower priced units.