It's really a matter of preference, style, and intended use. For what it's worth, I have fast primes and external flashes, and if I have to choose between them I pick a fast prime.
I love my 50mm 1.4, it can shoot in dim lighting without flash and produce wonderful looking pictures with diffuse backgrounds (which a flash won't give you). However, shooting at large apertures is best for single person subjects or small groups in a line perpendicular to the lens. If you want to shoot groups indoors, then flash is the only real option as you need to be at f4 or smaller to keep everyone in focus.
If you're shooting mostly groups indoors, a dedicated flash can give great results if you bounce it off a ceiling or wall. The direct, deer in a headlight look won't change much with a 430ex vs onboard flash unless you bounce the flash off something (hopefully something white). This takes practice but is worth it for the results. Though keep in mind it will make the photo look different as you are re-creating the light environment.
Going back to my pick, a fast prime, you can always use the on-board flash for a little extra punch / fill light. If you do find yourself needing to shoot a group, try using manual camera settings that only meter 1-2 stops dark and use the pop-up flash for fill light. When I do this, I like using the flash exposure compensation to turn down the flash power a bit (~1 stop). This keeps the flash from overpowering the ambient light.
Hope that helps. I'd be happy to share further thoughts depending on what you plan to use it for. Above all, have fun and experiment.
maven3 (rep: 5.2k) posted Nov 03, 2010
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