Gas Grill vs. Burner for Paella
I make paella regularly.  I am either using a paella pan that fits into the top of my Big Green Egg, or a slightly smaller paella pan that I can use on my indoor gas range.  Given your two choices, I would say that both would work just fine.  You may have more direct control over the heat with the Iwatani, since the flame will be directly on the bottom of the pan, but that control (vs. the Weber) might not be that critical. If convenience is a factor, let that be your guide.  Which one is easier?  Otherwise, you could also build a brick or stone fire pit and cook directly over wood coals.  I find the smoke and the spectacle of an outdoor fire add nicely to the festivities of paella.