It really depends on if you want the $$ and use up some miles or spend the $$ and use SWU's (assuming you will not use them for other trips). Ultimately, it's about your perceived value for the miles versus cash on hand.

Regardless, ensure upgrade availability is there if you go the SWU route.

Keep in mind you cannot use SM to book AF La Premier (first) awards - you can only book business class on AF.