Why the different wing and tail designs in similar sized jets vs turbo prop?
Well, for starters, the speeds are actually pretty significantly different. The Cseries is almost 25% faster than the Q400. Swept airfoils are much more efficient at those higher speeds, as are jet engines.

The Q400 is also a wildly stretched version of the original DeHavilland Canada DHC-8 (Dash 8), which was itself based on the 4-engine DHC-7 (Dash 7) STOL airliner. DHC has a lot of history building short takeoff and landing aircraft with pretty extreme performance, and a lot of that colored the design of the Dash 8. What you see today in the Q400 is the result of about 70 years of backcountry aircraft design, applied to a commuter airliner.

The Cseries, on the other hand, is a clean-sheet Bombardier design meant to compete with the E-jets and the smallest 737s (and 717).

Hopefully that answers your questions!