I've had muscle spasms in the lower back& told two diff. things. 2 docs said to apply ice & 1 said heat. i'm confused, which is best to reduce?

Ice right after: And heat later on.

Muscle spasm: Actually both are right in terms of using these modalities to treat pain or spasm. Generally ice is reserved for the first 24-48hrs after your symptoms develop and heat is used thereafter. There is no concrete scientific evidence that one is much better than another or that it makes a difference to use them or not. If there's inflammation use cold before heat, thereafter do what feels good.