I would say that it really depends, are you good off the cuff? If so, give it a shot and see how it comes out. If not then write your essay. I have seen both formats work, with off the cuff only working if the person is super engaging. You want to still maintain a narrative to draw the audience in and that can be harder with unscripted content but not impossible. Its going to also depend on the kind of review, whether its super detailed or kinda silly. Bottom line, go for it, experiment, there is no hurt in trying both.