I think I may be the best person here to help (see my sig)
I have the pleasure of driving a CMax Titanium Powershift.
I too would have liked the greater room in the Grand - and they do drive exactly the same, but due to budget I couldn't afford one with the options I wanted.
I went for Privacy Glass and the Convenience Pack, and haggled them down from around £600 to just under £400. So the total I paid was a smidge under £2k (£1595AP + £400)
The Active Park Assist is actually pretty good, it can be used on both offside and nearside parallel parking but can't 90deg park. I don't get much chance to use it up here as I have a driveway, but whenever I visit my in-laws in London (which is a lot) I use it all the time - and it's definitely beneficial when parking next to high kerbs. I've managed to park in a space just 6" longer than the car, so it does work!
The main reason for getting the pack was the electric mirrors - as I do use that feature a lot down the narrow roads where I live.
I would have loved the Titanium X pack for the heated seats but to be fair the seats are very comfy - my wife has just driven 1000 miles in a week and didn't complain once - I just couldn't justify the extra £2k.
If you live around here, I'll happily come and let you have a play.
If you've already driven the CMax, I wouldn't worry about buying blind, it really isn't that much different.
Perhaps you should join the free owners forum if you'd like to ask more specific questions -
http://www.cmaxownersclub.co.uk/forum/#a-2010-c-max (I'm Pauliboo on there).