Definitely prefer the theatrical version
I've been hearing about this supposedly superior cut of Exorcist III for years. However, now that I've seen it (or something close to it), I can totally see why they wanted to reshoot. The theatrical version's climactic scene, for all its schlock content, is at least a climax. This DC just...ends. It's totally anticlimactic in the worst way. There's no build up, no tension at all, apart from the scene with the nurse at Kinderman's house. The theatrical might be a little over the top, but it at least ties its finale back to something akin to the original Exorcist's ending, namely Damien sacrificing himself as a means of banishing this demon back to hell.
Plus, I really prefer having Jason Miller playing Damien in between scenes with Dourif as The Gemini. It makes so much more sense, particularly for something intending to be a true sequel to the first Exorcist. I would never have bought Dourif as Damien. It just doesn't feel right at all. Maybe that's because I've been watching the theatrical cut for so long, but I suspect not. Replacing an actor with another actor for a sequel is rarely a smooth transition and this is no exception.
I also thought Dourif's performance in the theatrical cut was much better. He's far more sedate in this DC, which doesn't fit the character and his situation. The Gemini is supposed to have been trapped in a broken body for 15 years. That, on top of already being a psycho killer, makes Dourif's intensely manic performance in the theatrical much more appropriate. He should be completely on the edge after being so stifled for all that time.
Anyway, that's my two cents. Still a quality Blu-ray and an excellent transfer of the film. I'm so glad we have this now.
If you disagree, well, so be it.