  Using density matrix renormalization group calculations, we compare results
obtained for the t-J, one-band Hubbard and three-band Hubbard models of a
two-leg CuO ladder. Spin and charge gaps, pair binding energies, and effective
pair hoppings are calculated for a wide range of parameters. All three models
have an insulating state at a filling corresponding to one hole per Cu site.
For physically relevant parameters their spin gaps are similar in size but they
exhibit quite different charge gaps. We find that the binding energy of a pair
of doped holes is of the order of the undoped ladder spin gap for all three
models. The main difference between the models is the size of the effective
pair hopping, which is significantly larger in the three-band model with
parameters appropriate for CuO materials than in the other two models.
