I would do the automatic machine if the doctor will go that route. They're set for a pressure range, and will keep the pressure as low as possible to stabilize the airway. As that changes through the night with different sleep stages, it'll adjust automatically. It's slick. That pressure adjustment makes for a more comfortable (well, less uncomfortable) experience, in my opinion.
Plus, as you said, you won't need to do that overnight sleep study, which is a bonus. You also won't have to do regular follow-ups to get the pressure tweaked, as you would with a fixed CPAP.
Just as a side note, most of these machines (both fixed and auto) have a "ramp-up" mode which is supposed to make things more comfortable, but I actually found it made me feel like I couldn't get enough air. I turned it off, and it's much better that way. Just an FYI for when you get yours, whichever way you go!