Originally Posted by alo

I don't have scientific proof, but my gut feeling is, there would be a small number of punctures that would seal in a tire and tube, but not in a tubeless tire. This is because the sealant would collect in the space between the tire and tube, forming the seal.

So sealant in a tube probably provides slightly better flat prevention.

Add to this, little mess normally, as the sealant is contained in the tube, and the ability to change the tire, while reusing the tube with sealant in it, instead of needing new sealant.