Consensus on this board is that a mile in an airline loyalty program represents a higher value than a HiltonHonors point.
So if your choice is either X miles or X points then you would get a better value chosing the miles.

Also several airlines will allow you to transfer miles to HHonors earning you 2 HHpoints per mile transferred. I'm not sure if NW allows these transfers, but eg. AA and UA do. So if you would earn X miles with your purchase, they can later be transfered to 2X HHonors points.
There are some limitations like not every airline program allowing this transfer and the fact that transfers usually can only be done in multiples of 5K miles.

Personally I always try to earn miles instead of points.

Edited to add: forget about transferring from UA, see here.

[This message has been edited by ql2112 (edited 03-20-2003).]