I wouldn't think there would be a significant difference, but I hadn't thought about the effects of condensation.
even so, I think it would just act somewhat like a diffuser, and scatter the beams, as opposed to absorbing/blocking/reflecting them. not certain though.
If it is just refracting/scattering, I don't think it would have a big effect unless you are using a focused light spread on a fairly deep tank. Optics really aren't my thing though.