They are both rhetorical. There is no vous but nous in these sentences so by using "you believe", you aren't exactly matching the meaning which is more like:
What would tell us (i.e. lead us to believe) that plants and bacterias aren't sentient too.
The second sentence says "Who tell us that..." but the meaning is equivalent.
An alternative translation might be:
What states that plants and bacterias...
Who states that plants and bacterias...
To answer to Emmanuel's comments, there is absolutely no doubt the question is rhetorical, i.e. a question that doesn't really expect an answer, especially a contradictory one, but is just used to make a point.
Here is the full sentence:
Qu’est-ce qui nous dit que les plantes et les bactéries, elles aussi, ne sont pas sensibles (elles le sont d’ailleurs, d’une manière ou d’une autre, j’en suis persuadé) ?