Are users downloading the cross tab/csv data of the viz or are they wanting to d/l a png?
They want a crosstab/csv.
Is there a way to do that?
Yes, you can create a custom d/l button, rename the sheet you want to default to print to begin with A (Tableau pulls alphabetically)...
This is a good demo done by Andy: The Greatest Tableau Tip EVER: Exporting CSV made simple!
I saw that article last night and I really liked it but I noticed right away that parameters and filters did not go across the csv export. Additionally, the trick about using ".csv" at the end of the name is not exactly the same result as using the "Crosstab" button.
Using the .csv trick, the columns come back sorted first Dimensions (alphabetically), then measures (alphabetically). Using cross tab, the all columns are sorted alphabetically together (no distinction between measures and dimensions). So if I want to control the layout completely, I can prefix all my columns with 001, 002, 003, etc. and get them ordered properly. Then I can use the export crosstab option and get everything formatted exactly the way I want.
I answered my question half-way accidentally. If you select the report you want "Defaulted" and then publish, it will automatically be selected everytime the report is rendered on Server.
Now, my only remaining question is, is there anyway to make the other report completely un-selectable?
Edit: See my other question here on how to make a report un-selectable.