The standard size (15") Challenger is very easy to control on all but the lowest pile, commercial glue-down carpeting. For this type of job, I would recommend a machine with wheels which are on the floor while the work is being performed.
The Challenger Titan (17") is available with wheels either up or down. The same size Orbitec Defender comes with wheels on the floor. I cannot comment on the Orbot, as I do not own one.