DMSO is the way to go, however there are references that state DMSO can be toxic to some cells, so do some baseline trials and see how they react. I have read that glicerol can be used in some cases as well.
ethanol is much more toxic than DMSO! Ethanol is a heavily cytotoxic compound, I experienced this during own experiments. Do never use ethanol in MTT assays, this is not reliable and falsifies results.
ethanol is metabolized to acetaldehyde by alcohol dehydrogenase. Acetaldehyde is strongly cytotoxic since it reacts with protein amino acid residues and induces protein and nucleic acid cross-linking. DMSO is not metabolized in such a way, although it has cell-differentiating effects in higher concentrations.