I am planning on having a 73 sport dart media blasted. I have a second set of doors that are in good shape that will be installed after the blasting. Before blasing, is it better to remove the existing doors with the hinges attached to the doors or leave the door hinges attached to the body ?

I would think there's some corrosion behind the hinges on both surfaces.
Getting the media out of the hinge pivots might be difficult; stripping is probably better for them.
Whenever you separate door hinges from the body, photo or mark adjustment locations.
That will save you some hassle with reassembly & adjustment.
Plan on a fair amount of time removing media from crevices.
I had a paint rep who hated soda blasting, claiming it created a primer adhesion problem.
I would think a thorough cleaning & de-greasing would remedy that, but never had soda done.

I had 63 ply fenders soda blasted. In the inside nooks and crannies the paint came off in sheets. I also heard insurance claims against a painted soda blasted car won't hold up.

the more taken off, the less work you will have stripping old paint and primer later

hinges off makes sense. I believe there will be a fair amount of metal work to be done after blasting so I don't think any painting would be done for at least six months, which probably means two years off in reality. I would think the correct metal preparation would cancel any effects of the blasting, especially over an extended period of time.

I had my car done with plastic shot, I took everything off. They went over it after with sand to prep for paint adhesion. Didnt hurt the metal anywhere except the roof that i have found. I read to avoid soda.

I sprung for a used DB-800 dustless blaster. We'll find out this summer if all the hype is accurate.