Not sure I understand the flash question either. CRHS, are you asking on-camera vs. hotshoe flash or flash vs. no flash?
The on-camera flash runs off the camera battery, so usage time will be shorter. As opposed to using an external unit that would run dedicated power.
On-camera flashes are limited to a couple [brightness] settings, whereas an external one will be much more flexible and have more range.
Since on-camera flashes are fixed, they can point in only one direction, whereas an external will probably have an adjustable head so you can bounce light, etc.
If you plan to shoot in locations where you'll need it, and if you can afford the couple hundred (or more) dollars, an external is worth it.
And if the question is considering natural vs. artificial light, that's easy: I use natural as much and whenever possible. Flash is an aid, a supplement, not a replacement (unless, of course, it's the only source available).