While the empirical and theoretical industrial organization literature has traditionally focussed on the growth rate of firm size, we take the position that profit rates, measured as the ratio of operating income to total assets, are economically more fundamental than logarithmic time differences in firm size, and hence should attain more attention in economic inquiry. The reason why we consider the profit rate so important from an economic viewpoint is that the economy is ultimately driven by the reallocation of capital, and investment decisions are governed by the rate of return per unit of invested capital. Additionally, profit rates exhibit peculiar time series and cross-sectional properties that make them appear an adequate quantity to study the complex interactions of competitive firms in a statistical equilibrium framework. Considering a sample of more than 500 long-lived publicly-traded U.S. companies, the purpose of this contribution is to explore the statistical features of profit rates, and to compare our findings to the properties of another very prominent degree of freedom for this problem: the growth rate of firm size. We find that the empirical densities of both quantities can be reasonably well described by the family of Subbotin distributions, but there are characteristic differences in their autocorrelation structures. Moreover, while there is a negative effect of size on the mean absolute deviation of growth rates, it turns out that neither location nor the dispersion of profit rates depends of firm size. In the second part of this paper, we employ the statistical equilibrium model of competitive firms proposed by Alfarano et al. (2012) to model the dynamic evolution of profit rates and show that this particular stochastic process reproduces the statistical characteristics of the profit rate time series. Moreover, we use the solution of the transient density of this process to estimate the diffusion coefficient, which allows us to measure the characteristic time scale for the dissipation of excess profits.