IR phototransistors are much more responsive (report light changes more quickly) than photocells. If you try to catch a washer free-falling through the hole with a photocell I guarantee you'll have problems -- even using a laser as a light source. Using a real world example I can say that you can pretty lazily drop your arm through the laser and the photocell won't give enough voltage swing for accurate detection.
The simplest solution would be to put a funnel under the hole so the washer ultimately drops through a restricted space. I'd still recommend using an IR phototransistor and a red LED. An IR LED is not required and makes things more difficult because you can't see it. The other advantage of phototransistors is that you can use them with digital inputs and won't have the problems with the Arduino ADC mux.