Important considerations, indeed.

Don't forget that if you would elect to do both, you have to factor in the time it takes to get to and from the stations in Rome and Florence (plus waiting time) - in other words, that 90 minutes actually ends up being longer.

Given the short time involved, I'd say that if you are more oriented towards visual art (painting, sclupture, etc) go to Florence. If you're more into historical structures, monuments, and churches, etc., do a "highlights tour" of Rome.