IR Reflectance Sensor for a Wheel Encoder - Analog or Digital?
I've used both of them, and I like the analog ones more. 

The reason is that with IR, there can be a significant amount of 'stray light' coming from the environment. If you have an analog sensor, you can adjust your thresholds and baselines to tune the sensor to your operation (you have a lot of flexibility). When I used it, I would take 100 readings when I turned the device on, use that as the baseline. 

With the digital ones - you're stuck if it's triggering and you don't want it to (you need to physically shield the sensor). 

They do use the same sensor in both of the devices, they just change how the phototransistor operates (cap+resistor for the digital version, just a resistor for the analog version), so if you really wanted to, you could change the operation of the sensor (if you have the right resistors and caps).