What the difference between special depreciation allowances and 179 depreciation

The special depreciation allowance applies only for the first year the property is placed in service. This is an additional deduction you can take after any section 179 expense deduction and before you figure regular depreciation under the modified accelerated cost recovery system (MACRS). Qualified property.

That is a huge question, but the simplified answer is that they are very similar.  For most people and situations, the Special Depreciation Allowance is preferable for Federal purposes.  One reason for that is because if you use Section 179 and then in a later year the business percentage drops to 50% or less, there is a "recapture" provision.  The Special Depreciation Allowance does not have  that (which is good).