In that case they would be identical....
I think i read somewhere that NaClO3 is stronger....but Im quite sure why.
If you are using sodium chlorate to oxidize something in solution, it might be stronger, due to the fact that it is more soluble than potassium chlorate. But by themselves in a solid state mixture (like fireworks for example) they would probably have the same power.
That being said, take my answers with a grain of salt, as I am not an expert in the 2 compounds.