Upvoted for this:

Step one is to throw out the question entirely. The most rational toothpaste does not exist, nor does the best toothpaste nor the optimal toothpaste. These adjectives are only applicable relative to particular goals, constraints, and contexts. Avoid the mistake of assuming optimality is a trait inherent to toothpaste, rather than a joint function of the toothpaste and who is using it. Similarly, the best programming language, the best footwear, the best way to write, and the best job are all under-specified.

(most "Rational X" posts sound like blatant other-optimizing to me). On the other hand,

Toothpaste seems like a product where users have essentially the same needs or fall into a small number of categories

Well... There are toothpastes which leave an awful taste in my mouth for a couple hours every time I used them, and I'd rather pay twice as much for a toothpaste than put up with that, but that doesn't seem to bother my parents in the slightest (as evidenced by the fact that they've bought some such toothpastes several times).

buying a tongue cleaner for $5

What advantages does that have over just scrubbing your tongue with your regular toothbrush?