Chukkas in shell are wonderful - I have them in whiskey, cigar, black. The patina on the whiskey and cigar is very nice. With pant leg down, they have damn near the exact appearance of a PTB.
I have no opinion on the suede chukka.
My experience has been varied. I wear a 9.5D Barrie, and I love this last for my feet. I couldn't fit properly into ANY Aberdeen last in a full-strap loafer (I tried 9.5D, 9.5E, 10C and 10D), but I fit very well into the Aberdeen monk (9.5E). To some extent, it varies on the model shoe, and of course your foot's dimensions. Because the last is very narrow in the toe, I don't think there's a ready formula like there is with some other lasts, and you prob need to actually try on the actual shoe to know for sure. Remember Barrie and Aberdeen are at 2 opposite ends of the last spectrum.
Which model are you exploring for the Aberdeen?