Section 179 strengths
Elective and asset-by-asset (you choose exactly how much to deduct). Cannot create or increase a loss — limited to active trade-or-business taxable income. Lets you fine-tune the deduction to a target income level (often the QBI threshold or a tax bracket transition).
Bonus depreciation strengths
No taxable-income limit (can create a loss). Available on used property as well as new. Applies automatically (you elect out, not in). Useful when you want maximum first-year deduction without regard to income.
Phase-down concern
Bonus depreciation is phasing down 20 percentage points per year — 100% (2017-2022), 80% (2023), 60% (2024), 40% (2025), 20% (2026), 0% (2027) absent congressional extension. Section 179 is a permanent provision (currently). For property placed in service in late years of the phase-down, Section 179 may be preferable.
State conformity
Many states do not conform to federal bonus depreciation, requiring an add-back on the state return. Section 179 conformity is more common but capped at lower limits in some states (e.g., California). Check your state's rules before relying on either provision for state tax planning.
Combined strategy
Most planners use Section 179 first to set a target income (e.g., just below the SSTB phase-out for QBI), then layer in bonus depreciation for the rest. The combination gives full first-year deduction with the income-targeting benefit of Section 179.
Where the IRS publishes guidance on this topic
The IRS publishes a layered set of free resources on most small-business tax topics: a relevant publication (usually one of Pub 334, 463, 535, 587, 946, 560, or 583), the instructions to the relevant form, the "Small Business and Self-Employed Tax Center" landing pages on IRS.gov, and the Audit Techniques Guides written for IRS examiners. Reading the agency's own materials is the cheapest tax education available. They are written in plain English, updated annually, and they reflect exactly the framework an examiner will apply if your return is selected.
How experienced filers approach this
Experienced self-employed filers and the CPAs who advise them treat this question as a recurring planning exercise rather than a one-time decision. They model the multi-year tax impact rather than just the current year, document the reasoning in a short workpaper that survives staff turnover and software changes, and revisit the analysis annually as facts and laws change. The discipline is not difficult — a half-day in January with last year's return, the current-year IRS publications, and a spreadsheet — but it is rare among DIY filers, which is precisely why it produces outsized results.