If you are going with the factory style 1970 dual longitudinal stripes, they are pretty easy to layout and paint. As others have said, if your car is outside a lot, they with probably last longer plus it will give you a more custom look if they are painted on.

On the other hand, if you are looking to do the upper body stripes like on a 71 or 72, they are harder to get right unless you order a stencil/mask kit.

Truthfully, it boils down to personal preferences. Me, I'd prefer if you bought some stripes from me but I can see the benefit of painting them on as well. Even though I actually produce stripes myself, I sometimes paint them on based on my "vision" for the car/truck I am building.

Of course the biggest con of painting on stripes is if you change your mind and want to do something different down the road.