Both the croissant and bagel are healthy for you but when it comes to calories the bagel contains less calories than a croissant of the same size.
A 3 inch bagel contains around 170 calories while a croissant of the same weight has more than 270 calories so that makes the bagel healthier for you than the croissant.
If you're on a diet then the bagel would be the healthiest for you to eat but if you're not on a diet then eating both the croissant and bagels are equally as good.
The main issue is in the calories but the croissants and bagels contain health benefits as well so your health can improve when eating croissants and bagels.
But just avoid overeating because overeating of anything can be harmful to your health and cause weight gain.