"~てくれませんか" is the most polite way to ask. Because Japanese would prefer a euphemistic way of speaking. It is equel "could you ~" in English. ~てくだい is equal "do something please" But case by case, If you are boss or a consumer, you can say both of these ways you like. If you are speaking to boss or someone you know, the first way is better. Sometimes they are interchangeable, but l think it depends on your situation.Have a nice day too.