I wish I could say that I spent a lot of time studying component data sheets and weighing the trade offs. But, it's just not true. My approach was much simpler and more straight forward. While checking the bins at a shop in the Akihabara electronics district I bought several different optical sensors primarily because they were cheap and I wanted to experiment with them.
One of the sensors I bought was the Toshiba TLP800A photo-interrupter infrared LED and photo-transistor. The LED and photo-transistor are housed in a common package with a 3 mm slit between them. In normal use a shutter, usually part of a small diameter disk, rotates through the gap interrupting the light from the LED
Using the TLP800A it was surprisingly easy to breadboard a test jig and circuit. The cutouts in Maxwell's chassis extend slightly past the outside edge of the wheels. Heavy tape was used to secure the sensor in position, and to fashion a simple shutter. The shutter was fastened to the wheel using two push pins (the wheels are black foam). A DVM monitored the output signal while the wheel was rotated manually to adjust the shutter to sensor positioning. Once the two were properly aligned, the photo-transistor output signal was connected to the micro controller and the test program ran automatically.
The test program was run several times on each servo and the results were extremely consistent - a significant improvement over the previous manual method. Timing data was collected for PULSOUT values ranging from 500 to 1000. The test program writes the results of every cycle to the DEBUG console in an Excel friendly CSV format. This makes the test data extremely easy to chart and analyze.