Points.
Makes you eligible for more bonus opportunities. You can always buy miles with your marriott points if you want to (at a better rate).
Example=
$3000 spent = 9,000 airline miles
OR with the points option:
$3,000 spent x 10 points per dollar (more if elite)= 30,000 points which can be exchanged for 10,000 airline miles.

see http://www.marriottrewards.com/awards/airrewards.asp