Which one is healthier?

I don't think either one is healthier than the other, it solely depends on your hair type and what you are using the product for. The heat of the blow dryer that follows is usually what's damaging your hair.

Personally, I think mousse is better because it doesn't make your hair as crunchy.

mousse is much healthier it allows it to breath.
gel covers, smothers, and saturates hair not letting oxygen get to it

I personally like mousse better, hair gel dries my hair out...