Yes, skibum, there is a difference.
First, prednisone is a steroid but an injection (an intramuscular injection) into your buttocks is not a spinal steroid injection.
The spinal steroid injection uses a steroid (most likely prednisone also along with a numbing agent) but the placement is into your back around the area of the herniated discs - not into your buttocks. A CT scan is required before doing a spinal injection so the physician knows exactly where to place this injection.
The spinal steroid injection is more targeted. But it also can not work or for those who are lucky it does.
Hope your sciatica pain (buttocks & leg) heals on its own. It's possible. Mine did.