what is the spec of your airflex pro ... is it two in series or two in parallel , i know u have the booster also .
nobody has done the definitive test that answers your question ... mainly because of all the variables that can effect the results .
In general two inch seems to preserve airflow on longer runs .
At 50 foot it may not have made such a difference on that day with that carpet , other factors maay have helped drying also , like wand contact on that particular carpet ... ambient temperature etc
I try and use two inch as much as possible , but if its easier to use 1.5 in a particular job , il use that ... or combine both .