E.g. If you set 70mph on ACC it will do exactly the same as CC if you are on an open road with no traffic.
BUT ACC will also detect traffic in front of you and, if necessary, reduce accordingly (to a distance you can set).
With normal CC, you would have to turn it off if you came up behind a vehicle doing 65. But ACC will adjust your speed to 65......and if the car in front then changes lane, your car will automatically speed back up to 70.
I don't know of a situation where you would want CC but not ACC? If there's traffic ahead, with CC you'll have to apply brakes which will disengage it. Ditto for ACC, with the added advantage of it slowing down if you want the option. If you want it to behave like CC, simply apply brakes.
On single carriageways ACC comes into it's own, you really just have to steer (and Lane Assist will do most of that if you let it!)
The only issues I have with ACC (and therefore would prefer normal CC in these circumstances) is when on a dual carriageway / motorway and approaching a slower vehicle with moderate traffic around you, the car will slow down a bit too early for my liking, if you pull out to overtake the moment the vehicle in front appears on the maxidot then you are in the overtaking lane for longer than is ideal, this is no problem if you are travelling much quicker of course but is not ideal if you are only going a few mph quicker, yes I know i can just put my foot down but that kind of defeats the whole point of CC.
If it is ACC or CC then I would not spec it, however I do like most of what ACC does.
I've found no way of turning the Adaptive element off. 95% of time it's perfect, and am learning those 5% of times when I'm required to think! If a Hazard Situation is developing in front of me, I turn it off.
I'd say that at the end of a day driving with sensible ACC use I'm much less stressed than without it.