Yeah, a long, steep hill is definitely better - because the average speed of the run will be higher than a coastdown on level ground. So aero changes will have a bigger impact.
I was pretty excited when I
thought I had found a decent hill for testing , but I ended up not using it: the speed really was too low (maxed out in the low 30's), plus there's a railway crossing right at the bottom. I did have to jump on the binders for a train once.
Crossing the tracks at 30 mph when there was no train coming was pretty rough too.