That depends on what you mean by "more powerful" and in which context.
Cheats are very powerful, they're hard-coded in Warcraft (?) and rather hard to prevent.
Of course: there ARE ways to prevent cheats, but some are rather extensive.
On the other hand, because of JASS you are able to create an entire map. You can also create new cheats, or disable already existing ones with it ('disable' means something more like 'detect and avoid', as actually disabling them is not possible). When you consider all that, cheats can't do much else besides what they're meant to do, while JASS can go beyond that.
Why exactly do you pose this question? It's hard to formulate an answer, as everything is relative (cheats are more powerful than JASS when it comes to what they're SUPPOSED to do, JASS is more powerful than cheats in terms of versatility).