Will external flash make a difference?

I just purchased a Nikon D3100, and I was debating on whether or not to get the nikon sb 600 external flash. I have been told two different stories. Some say it will make a difference and others say it won't. I personally have seen in some cases where it both has and hasn't made a difference. I mainly do outside pictures, portoraits, and some inside, and night time photography.

For indoor portrait photos it will make a huge difference.

If they don't make a difference why do All pros use them for flash photography?

An external flash can do wonders to your image quality, but you need to know how to work with it.
In most cases, pointing it half way up to the the ceiling or sideways to the nearest wall will create a soft lighting effect and take away those harsh shadows you usually get when using the camera's internal flash.
In short, I strongly suggest buying it, and taking the time to learn how to use it.

Why don't you give yourself some time before spending $200+ for an external flash and see if you really need it.

From your description, it doesn't sound like you do a lot of flash photography. With the improved higher ISO performance of many DSLRs today, you may not use the flash frequently. For some outside pictures, the built-in flash should be fine for fill-in flash.

Personally, I don't have an external flash because I don't do a lot of flash photography. If I did, though, I'd definitely get one. Those who say it doesn't make a difference, are incorrect. My guess is that they don't know how to use one correctly.

It can make a big difference. The SB600 is a nice flash. Even though I don't use mine much, it is invaluable when I do need it. People with point & shoots are always amazed that my pictures look so much better than theirs when shooting flash. The TTL metering system of modern flashes is magic. It's almost fool proof unless you don't read the manual.

For indoors, learn about "bounce flash" techniques. This is where you aim the flash at a white ceiling or wall to give even lighting. You can remove the flash from the camera and hold it off to the side if you use radio remotes or cables. I like using a bounce card to give a bit of highlights.

For outdoors, flash is nice to use during daylight if you want to soften shadows. Learn about "fill flash". You can even gel your flash to match the color of the flash to sunlight. My flash probably gets more use when shooting in bright sunlight. If shooting outdoors at night, any flash will look pretty ghastly unless you use modifiers and/or move the flash off camera.

Some interesting starting places:
http://www.strobist.blogspot.com/

I shoot a lot for work. Indoors, outdoors, day, night… A LOT. You ever tried to take pictures of a fire scene? Try that some time without flash. How about under a car at noon? Try it… Put something like a shoe under the middle of a car. Broad daylight. Now take a picture of that shoe… So that you can easily tell what kind of shoe it is. Not a silhouette or blur, a clear picture of a shoe. You can't do it without flash. No matter what sort of high-ISO you try and use.

When I take my camera out of my bag, I put the flash on the shoe. Every time. Outdoors in daylight? Indoors at night? You bet I do. Every time. If it's really dark, I put the 2nd flash in my pocket.

For my personal gear, I have small, medium, and large flashes to choose from. I usually go with the large. I may not need it, but I will have it if I do.

Get the flash. Once you get it, learn how to use it. Once you do, you will see just how flash CAN make a difference in your photography.

Yes it will make a difference even with outdoor portraits. How much of a difference depends on your skills. I'd say get the hang of that d3100 and then buy a flash

Both people are correct.

Sticking an external flash on the camera and setting it on auto will not make a difference. Unfortunately I found this out the hard way a few years ago when I bought my first external flash (thinking it would somehow make my pictures better - it didn't).

If you take the time to actually read the owner's manual that came with your flash (and read the section on flash photography in your camera manual), then yes your pictures can come out better with an external flash. The important thing is knowing how to use it to it's full advantage.