Probably the main thing about this answer is I am not prepared to be drawn into a pointless religious debate over whats better.
In general, Tents are better at keeping water out but heavier and in some people opinion, more complicated. Tarps are lighter to carry and simple (In theory) to set up, if you have the skills.
Some people find tarps are 'good enough' at keeping water out, like their other features (weight, simplicity, open to the wild etc) and prefer tarps. Others prefer tents, feeling that the extra weight is worth the perceived advantages.
As far as getting flooded in rain, it comes down to your bushman-ship. A skilled person won't have any problems, will select a good site and position the tarp so rain is not a problem. In the same situation, a less skill person may find themselves soaking wet and drowning in an inch of water. The tub floor of a tent is more of an advantage to a less skilled person.