Short answer: when in doubt use ice.

Long answer:

Ice will:

Reduce swelling (which can stretch damage tissues).
Constrict the blood vessels, stopping bleeding (bruising) and reducing muscle spasms.
Numb the pain and slow the accumulation of irritant molecules sent by the body to combat infection.
ice pack
Ice pack
Thus for any acute or chronically re-injured tissue (ie tennis elbow), use ice. (10-15 minutes on, 1 hour off, repeadedly)
Heat will: increase the blood flow and soften fibrotic scar tissue. However, it will also increase inflammation which can inhibit the tissue healing process. So even though heat can feel good on an acute low back injury, it is not the best therapy. Use heat on chronic achy areas or chronically stiff areas to soften them up. (up to half an hour of heat, then get up and move).