Difference between ATR and a Dash 8? For me,as a passenger in NZ - I fly a route from Dunedin via CHC to Nelson that sees me changing from an ATR to a Dash 8 Q300. So I do see the difference. The seats feel about the same - it is the ankle room. The Dash 8 fuselage curves in at ankle level, so the person in the window seat will notice that. The ATR sidewall goes straight down. (This would be whether you are in a Q300 or Q400.)
On NZ routes, the Q300 and the ATR have the same time shown in the timetable. Some routes north of CHC to Hamilton or Tauranga, or Dunedin to Wellington would have a small advantage if Q400s were used.
Would be nice to have some Alliance Fokker 100s in NZ.