Numerous models have been proposed to account for gas-solid reactions where conversion does not occur at a sharp moving front and where the simple shrinking core model is not applicable. Here we compare the simplest representations of the two broad classes of such models : the grainy porous pellet model of Szekely and coworkers for the reaction of porous particles, and the crackling core model of Park and Levenspiel for the reaction of dense particles. The phenomenological view, the conversion-time behavior and the size dependency are markedly different for these two models. Experimental methods for evaluating the parameters of these models and for choosing the right one for the system at hand are discussed.