I've not actually been in a Flex, but my wife's MKT is based on the same platform. The rear of the MKT slopes back unlike the Flex, which is more of a right angle. For an adult, the headroom in the 3rd row can be a bit uncomfortable. I suspect the Flex would be better in that area. I drove my wife's MKT on a roadtrip with the family a few weeks back and it's a great highway cruiser! We don't put a lot of miles on our vehicles (I just turned 8000 miles on my 2011 SHO that I bought in June of 2010) but no issues so far.
-Rod