Slabs or prehungs depend on your contractor and his abilities. Areal lumberyard, not a box store is a good place to start in my opinion.

I'm not sure many craftspersons have the experience or the tools to hang slab doors well.

Hanging prehung will be much faster. And there are a lot of folks adept at installing prehungs.

Most interior slab doors are hollow core---meaning little sound proofing. Solid core doors will be better for that.

Second the advice about a real lumber yard for better selection and quality.

Around here there are window and door specialty shops that display many more varieties of doors than the lumber yards. Either such a shop or a lumber yard will offer you better quality choices than the Borg.

We used solid core doors from Simpson that were relatively inexpensive and seem pretty good at blocking sound. They weigh a ton, and have a nice substantial feel to them. They lose points for greenness, as they are MDF. Would have loved real wood doors, but they weren't in the budget. Apparently there are foam core doors that might be good for sound

Hanging a slab in a existing frame isn't the hardest thing in the world, it just takes a little attention to detail. Since you're back to the studs, pre-hung doors are probably going to be quicker.

Have you talked to your contractor? Have him give you prices both ways and ask what he prefers.