Matt,
I think the US Open setup throws a whole other set of variables into the mix as far as difficulty, so I'm not sure the Mickelson case is where I'd lay my foundation.
For instance, let's say that the end of the fairway of 16 at Merion for the US Open is about 23-26 yards wide, with six inch rough on either side, which will probably be the case. Suddenly that "short iron" becomes a decision of whether to try to carry 150 yards of quarry on the approach, or a chip to the fairway.
The new back tees on 17 make the hole 250 yards...18 plays 505 with a 250 yard uphill carry out of the quarry. On 16, although the approach shot will be about 150 yards for most assuming it's not a driver hole, the green sits significantly above the fairway, and it's also probably the most devilishly difficult green on the course with the exception of #5.
So course setup for extreme tournaments should probably not factor into the equation here.
Instead, from the back tees, I'd ask you this. Which is easier for the average golfer to make 3 bogeys...the final 3 at Winged Foot, or the final 3 at Merion?