Hello.
We generally recommend the digital versions of our QTR reflectance sensor arrays. They perform better than the analog versions under poor conditions (such as low ambient light) and have slightly farther sensing distances. Also, the digital version of the sensor is analog with respect to time. Specifically, it requires a digital I/O line to charge an output capacitor, then measures the time it takes to discharge the capacitor through the phototransistor (so you get a lot more information about the reflectance than just 0 or 1).
Some people like to have one sensor directly in the middle of the line; however, since the line in your competition is 2cm wide, it will not get lost between two sensors on the array, so it should not matter. The QTR sensor array will be able to detect and give a reading showing the line is between the two sensors.
Using the QTR-3RC in the middle with an individual sensor on each side is a viable option. As long as you are using the same type of sensor, you can use our QTR reflectance sensor AVR library to read your 3 separate sensor boards as a 5 sensor array. Each sensor is still read individually with the 8 sensor arrays, so you will not really be losing performance by using the 3 different sensor boards.
-Brandon