When I get rashes, the best stuff I've found is a hydrocortizone cream. Sometimes a benadryl cream or the tablets help relieve the itch as well, but the tablets cause drowsiness.
Prednisone is a nasty nasty drug. I'd try to avoid it at all costs. Causes all sorts of issues. They usually only provide prednisone if the rash covers 10+% of your body or it is on your face.
Most common side effects according to drugs.com:
"Difficulty sleeping; feeling of a whirling motion; increased appetite; increased sweating; indigestion; mood changes; nervousness.
Read more:
Prednisone Side Effects | Drugs.com"