I've flown Dash 1/300 and am now on the Q400 in Japan. I've never flown the ATR, so I won't try to compare. Here's my 2 yen:
Q400 balance - here we regularly have 74 pax and 100lbs freight, so we fly with about 3000lb ballast in the aft baggage compartment, and that solves all the balance problems.
Q400 reliability isn't bad, but it needs quite a lot of attention and preventative care from maintenance, especially compared to the earlier models.
Dash 100 has good field performance (200 is even better), but isn't quite STOL. A critical field is about 2500ft.
All dashs are tough to land nicely. You have to be perfectly aligned with the runway and put it down gently to be smooth (a wet surface helps a lot). However, once you've got it on the runway it sticks, and stops quickly.