I have made jelly using both Jello and agar-agar seaweed.
Jelly made with agar-agar is firm. Using concrete-pouring terminology, the agar-agar jellies that I made would have a slump of more than a foot. Whereas, my jellies made from jello would have a slump height of only about 4 inches.
That is, I estimate that I could make a 1 cubic foot block of jelly from agar-agar and it would still hold, whereas the the max size of a jello block is only 4 cubic inch before it starts crumbling. Just an estimate, have never tried making a cubic foot block of agar-agar.
Is there a way to firm up the jellies made from jello? Would adding tapioca flour help? But adding tapioca flour would destroy the pretty translucence of jello.
Also, agar-agar has a nice firm chewiness whereas eating jello is like eating yogurt.