To swap control and command, add the following to your ~/.Xmodmap file:
remove control = Control_L remove mod4 = Super_L Super_R keysym Control_L = Super_L keysym Super_L = Control_L keysym Super_R = Control_L add control = Control_L Control_R add mod4 = Super_L Super_R
The above current works for both an Apple laptop and an Apple keyboard.
Also, to use Command+Tab to cycle windows, make sure to update keyboard shortcuts to use Ctrl+Tab instead of Alt+Tab (for Xfce, Window Manager -> Keyboard). All other keyboard shortcuts should come through (from the Control to Command swap perspective).