if you don't itemize, it's hard to think of a situation where filing separate would be better. even if you do itemize the only its usually medical expenses that will be higher if incurred by the lower income taxpayer. however, under the tax laws, if one of you itemizes both must itemize. another situation would be if one had a high amount of municipal income subject to alternative minimum tax. (AMT)
the only way to be sure is to compute it both ways.
