This paper strives to merge two strands of the literature. The first group of papers compares ad valorem and unit taxes in a tax competition framework in terms of welfare. The second group of papers regards capital income taxes as a conjunction of taxes on pure profits and taxes on capital income. We find that, given decreasing returns to scale, there always exists a level of the share of deductible capital costs strictly smaller than one, such that for all values larger than this threshold, an ad valorem tax regime unambiguously Pareto-dominates a unit tax regime.