Should I take the Section 179 special depreciation allowance?

Are you a Sole Proprietor (your business is not a Corporation or a Partnership)? 

You first need to determine if you want to use the Standard Mileage Rate or if you want to use Actual Expenses.

The Standard Mileage Rate is easier to use because there are less things to keep track of.

Vehicle with very high business miles usually benefit more from the Standard Mileage Rate.  Vehicles that have very low TOTAL annual miles usually benefit more from using Actual Expenses.  If it is in-between, it varies, but quite often the Standard Mileage Rate ends up as the better long-term option.

If you use Actual Expenses in the first year, you can NEVER use the Standard Mileage Rate for that vehicle.  You must continue to use Actual Expenses.

Now we'll start getting to your actual question.  Section 179 and the Special Depreciation Allowance are two different things.  Both are part of Actual Expenses.  If you use the Standard Mileage Rate, both Section 179 and the Special Depreciation Allowance don't apply.

Is the vehicle under the 6000 pound threshold?

*IF* you want to use Actual Expenses (and never use the Standard Mileage Rate for this vehicle) and the vehicle is under the 6000 pound threshold, I suspect using Section 179 and specifically electing OUT of the Special Depreciation Allowance will be what you want to do.

However, if you expect the business percentage to drop to 50% or less before you sell the vehicle, you may not want to use Section 179, the Special Depreciation Allowance, or any other accelerated depreciation (such as 200%DB).  That is because when the business percentage drops to 50% or less, you will need to "recapture" some of the depreciation and that can be complicated.