To me it's six of one, half dozen the other. Removing the old jamb is going to take some time. On the exterior there's usually old caulk that needs to be cleaned up. Ditto for the threshold. Then you'll have to install the new jamb and plumb it. Interior you'll need to remove the trim, hopefully not damage it or the surrounding wall, then re-hang it and putty the nail holes. If the jamb and exterior trim is in good shape, I don't see a lot of advantages to buying a pre-hung door. With a slab you'll have to mortise for the hinges and drill for the lockset. In terms of overall time, it's probably about the same. Because you'll have the old door to use as a template, it's really not difficult to transfer the marks for the hinges to the new door slab. Then you'll just need a sharp chisel. If you have access to a router, Porter-Cable makes a spiffy single hinge template that will speed up the job. They're available in home centers, woodworking stores, and online for ~$35. Drilling for a lockset isn't that difficult and almost every home center has some kind of jig to speed up the process. Just measure carefully. "Ask This Old House" did a segment where Tommy installed a new interior door slab. It gives a good overview of the work involved. If you're using a wood door, don't skimp on finishing. Use high quality finishes and follow the instructions carefully, making sure to seal all six (6) sides of the door. Mike

Hanging a prehung is almost always easier for a DIYer than fitting a slab to an existing jamb. I recommend using trim head screws instead of nails since adjusting the shims is much easier. Basic quick and neat prehung procedure: Shim hinge side plumb(that is straight vertically). You need a long level. Remember to shim the bottom up temporarily(for flooring/ventillation clearance). Just make sure the edgesof the jambs are even with the walls. Next, install the top of the jamb so there is an even gap when the door is closed. Next, install the latch side so there is an even gap with the door. Install trim.

And if any part of the jamb is proud of the surface, you can always relieve the back edge of the trim if it is a problem, or use a plane to trim the jamb a titch after it is installed. The little bit of extra is there for a reason. You would not want to have the jamb a hair undersized, Not all walls will be perfect and fully plumb in all areas, so this gives you something to work with. Don't worry about this: it's a non-problem.

We are replacing all of our interior doors as the original 1959 doors have seen better days. Our installer looked at all of our jams and only recommended replacing the ones where it is a double door (longer story and I am too lazy to type it out). So that is what we are doing. He has replaced about 75% of them. He will be out next week week to complete the project. We only purchased solid doors for entry into a room and hollow for closet doors. We started this process literally days pre pandemic. We were waiting on additional doors as the manufacturer had shut down for a few months (all doors are custom fit).