After some experimenting with both mediums, I have to say spray painting (although technically you need both).
Not sure what others do, but I found taking figures completely apart, and spraying them as needed works great for outer pieces, then using scotch tape or masking tape (or foil held down by masking tape for larger areas) for more detail.
But thats mostly because I'm rather untalented with a paint brush, which you still need to do smaller details.
Sprays are the way to go, but make sure you get the good hobby stuff (and not the clear's, they are for radio controlled car bodies), as the $3 sprays from Sueprcheap are great for spraying metal or wood, not so good for plastics. (do not adhere well).
Looking For: Wreckers Saga TPB Collection (with Requiem)