Neil
I'm not qualified to recommend one machine over another, but I feel that dave has given an honest and excellent viewpoint.
I would add though that for any portable machine with an adequate working capacity, if a job demands, lets say 12 buckets of water, in real world terms, it doesn't matter if you fill/empty the machine once, twice or 4 times. For lower capacity machines, you will be filling/emptying whilst the next room is going through the dwell period of it's preparation, so no time is gained or lost with greater/lesser capacities.
Safe and happy cleaning

The Ken