I've had my girls in both, with a similar situation as you since they're only one "grade level" apart.
With no other information about the specific classes, teachers, philophy of the school, etc, I'd go with gymnastics. Gymnastics is JUST gymnastics classes. With dance lessons, even for preschoolers, there's usually a performance at the end of the year. This involves overpriced, uncomfortable costumes you can't pick out and you're screwed if they order the wrong size for your child. The recital could be late at night, after your child's bedtime, with no way to comfort your children immediately before or after their class' performance. Also, my daughters found that dance classes got boring when they stopped doing "general dance stuff" and focused almost exclusively on "recital practice."
Many of the issues I experienced with ballet class could be completely avoided with a better teacher and better school director- but none of these things are even a possibility with a gymnastics class.