Speed and thrust are both good in proper relationship. Your airplane has a preferred speed at which it flies and maneuvers best. What you want is a prop which will maintain that speed through maneuvers, accellerating while going up and braking while going down. Generally more diameter and less pitch is good. The Rascal is not a big airplane and the lighter you build it the better it will fly. My guess is that you'll like the 7 x 4, but one has to fly and try.

What you want is the airplane flying at its happy speed on lines of a length which gives a slow enough lap time that you can comfortably stunt the airplane, while providing plenty of line tension. Give me some RPM numbers on your props and I'll recommend a starting line length.