In my experience sand, and alumina are a little agressive and comes down to the skill of the operator in not removing to much metal, and minimising heat build up. A safer option is to use sodium carbonate to blast with, it is nice and gentle. Plastic beads are also reasonable soft, may struggle to remove the rust. Glass beads are more agressive than Sodium carbonate but less so than the sand and alumina.

I would ask around your local club or someone who has restored a car in your area to find out who is good at the media blasting, and then talk to them as damage or poor work can be done when using any type of media.