I have flown the ATR 42 and 72, as well as the Dash 8 400. From a pilot point of view the ATR is much nicer to fly and the logic of the avionics/FGS /A/P is far superior to the DAsh 8. Also the build quality of the ATR is far better than the Dash 8.
The Dash 8 400 is faster, with a cruising TAS of 360 kts, and climb rates of 2500-3000 fpm at lower levels.
Most people i know that have flown both, prefer the ATR, but the accountants like tha Dash 8.
Hope this helps answer your quesyion