Oh, you wanted the answers
Well he explained that the EPA index also factors in ozone and other pollutants which are not particulate matter, therefore the rating systems are different. I guess it's like comparing apples and oranges. He did mention that the particle count readout from the DC1100, since it counts each individual particle, would be more precise in measuring particulate matter than the EPA AQI which is a rough guide.
The DC1100 measures in particles per cubic foot / 100. So when you read the display, multiply by 100 to get the particles per cubic foot. I asked about placing the meter in a windy place, like a car with the windows open. He was a bit surprised that I would be considering that, and he did mention that it needs to be plugged in to a standard 120V outlet, but he said it doesn't matter if wind is blowing into it, the accuracy of the reading wouldn't be affected by that. (To satisfy your curiosity, I was thinking of measuring the air quality near some of our manufacturing plants and agricultural operations in the community) This also implies you could place the meter near a fan without worry.
As far as which model is best, he said the standard one is generally the choice for home and office uses - unless you're particularly concerned about the smallest particles between 0.5 and 1 micron - or you're a contractor who needs to go by the PM2.5/PM10 calibration standard.
I might as well mention here that I just ordered one - standard model - can't wait to take some measurements when it arrives! (Hope the results aren't too scary )