Another vote for the small wood dolly. Yes it needs a relatively clean floor. And the nuts holding the casters on should be secured with Locktite after a while.
And a negative vote for the similar flat plastic dolly. Don't bother. Like dragoneggs, mine broke the first time I used it. I took it back for an exchange and broke the floor sample in by half stepping on it. (180 lbs, its rated 200). Got another wooden one instead. Maybe the plastic one would work for a patio potted plant or something.
I noticed a professional piano mover used a pro-quality twin of the small wooden dolly to move a heavy upright piano. It's easier to tip the small dolly back on two wheels to turn a corner compared to a larger dolly.