Question for Ansoft Designer

If be asked to use transformer to match the impedance, how to determine which kind of board should be applied? Only depend on the requirement on Er? Any other difference bet. them?

Let's say, 'MS-RT_duroid 5880 (Er=2.20) 0.010 inch, 0.5 oz copper', or 'MS-FR4 (Er=4.4) 0.030 inch, 0.5 oz copper', etc.