I've seen good players in the Amateur Championship at Sandwich and experienced Deal with my own bogey-golfer's game. So take this for what it's worth...
I agree that Sandwich is the greater driving test, and in my opinion very much so. Then again the rough was up at Sandwich but not so at Deal the week I was there so perhaps that would even things up a bit. Still, there are more contours in the landing areas that much be accounted for at Sandwich.
I also agree that the greens at Deal are noticably more confounding to any level of player. As mentioned 12-17 are a remarkable set of putting surfaces--well actually the twelfth is sort of sui generis and perhaps not as much difficult as just neat and interesting--offering perhaps more opportunity for rewarding well-placed tee shots and complicating approaches from out of position.
I'll venture two guesses on this subject. One is that the reason Deal gets so much harder in the wind (for strong players) is that a modern aerial power game can negate most or all of the situations where a "good approach shot" (so called) bounds off into trouble. Enough breeze is necessary to remove that approach from the hands of elite young players, otherwise they can to a great extent ignore angles and just hold greens (or even sections of greens) with height and spin. Put enough wind out there to make a +3 handicap college-age player approach Deal with the trajectory I have to use and there's a lot of uncertainty about the outcome of approach shots. And as others have pointed out, the back nine in particular exhibits a dearth of flat 20-foot putt opportunities.
My second speculation is more of a long shot, just a stray thought really. Is it possible that even in elite competition there's something about the relative visual intimidation of the two courses that makes Sandwich seem more respect-worthy and this leads to the mistake of players attacking Deal somewhat more aggressively than optimum whereas they tack their way around Sandwich more conservatively? Frankly, at first viewing one might think that Royal Cinque Ports can be readily overpowered. I don't believe that is necessarily true but great portions of it seem there for the taking with the possible caveat "except for a few holes coming in". On the other hand, Royal St. George's is gorgeous and awe-inspiring from start to finish and many of the tee shot requirements serve as constant reminders of how ones round can slip away if too many tee shots drift into that long grass waving in the breeze.
I'm pretty sure that at the Amateur some of the less experienced or first-time competitors made that mistake vis a vis Princes. I think it's a perfectly natural tendency to look at the two courses (side by side, no less!) and think "OK, there's the tough one and over there is the not as tough one". In fact, due to the narrowness of the fairways and difficulty of the rough Princes "resisted scoring" pretty much as well as its illustrious neighbor in last year's championship. To my eye, Deal looks even more managable than Princes even though historic scoring seems to suggest otherwise.