  Different holographic dark-energy models are studied from a unifying point of
view. We compare models for which the Hubble scale, the future event horizon or
a quantity proportional to the Ricci scale are taken as the infrared cutoff
length. We demonstrate that the mere definition of the holographic dark-energy
density generally implies an interaction with the dark-matter component. We
discuss the relation between the equation-of-state parameter and the energy
density ratio of both components for each of the choices, as well as the
possibility of non-interacting and scaling solutions. Parameter estimations for
all three cutoff options are performed with the help of a Bayesian statistical
analysis, using data from supernovae type Ia and the history of the Hubble
parameter. The $\Lambda$CDM model is the clear winner of the analysis.
According to the Bayesian Information Criterion ($BIC$), all holographic models
should be considered as ruled out, since the difference $\Delta BIC$ to the
corresponding $\Lambda$CDM value is $> 10$. According to the Akaike Information
Criterion ($AIC$), however, we find $\Delta AIC$ $< 2$ for models with
Hubble-scale and Ricci-scale cutoffs, indicating, that they may still be
competitive. As we show for the example of the Ricci-scale case, also the use
of certain priors, reducing the number of free parameters to that of the
$\Lambda$CDM model, may result in a competitive holographic model.
