In the bad old days when this market sector was mainly a BAe748 -vs- the Fokker F27, it was high wing -vs- low wing. Low wing meant a more resilient undercarriage without overlong legs, more suitable for rough strips (hence the late life third-hand success of the 748 in Canada). High wing meant you could have bigger props and easier cabin/freight access, but more of a nuisance to maintain the engines. Fokker normally outsold the 748 by 2 to 1, some years more. Notably the two better sellers of recent years, the ATR and the Q400, are both high wing, which probably helps with ever more powerful engines and thus ever bigger props.
Closest to the ATP was the Saab 2000, which was also a bad seller, but generally seems well regarded. I took one a few months ago from London City to the Isle of Man, where it taxied in beside several rather down at heel ATPs that the sole remaining large (cargo) fleet operator of them maintains there. It was a bit of a contrast looking at the state of the two different types, built in the same year, from the terminal ...