The end results would be very similar, both do an excellent job, both have their pros and cons too. With the Scrubba tool you are cleaning a pass at a time, slight overlap and then another pass and so on, it means you are walking up the floor after every cleaning pass and then down again with the next pass, same for drying passes. Of course you can keep the passes short if you wish but I always liked to keep the passes as long as possible, seems to do a better job.
With the Rotovac of course it is a rotory machine and I prefer to clean about a square meter at a time and then move on to the next square. Less walking involved as you can more or less stand still for a lot of the time.
I always liked the brushing action of the Scrubba as it can ''shake''the deep down grit out of the pile for sure. So on carpet I would give both the Scrubba and Rotovac 10 out of 10, the Alltec Rotobrush ( if they still on sale) 6 out of10