I always spray. The canned stuff is different to the stuff in the pots, being primer, it seems to adhere much better to surfaces, and the finish (if you spray right) will be more even than it's possible to get with paint. It also gives you an ever-so-slightly rough texture to paint on top of (helps the pigment to take), wheras paint from the pot is more smooth.
Always use spray primer! I think you can buy primer in tins but it's for much bigger projects than models.
Oh, and it never hurts to cover spots you missed with the spray with watered down black paint from the pot or black ink.