Pop-up Flash or Speedlight?

I have a nikon D3100, and I love to take pictures of people. I was wondering if it was worth the investment in getting a speedlight, like the sb400, or not.
Also, I'm getting a flash diffuser either way. Do you have any diffusers you recommend?

Pop-up flash is the worst! Avoid at all costs- will ruin your photos. One diffuser I hear rated very highly by a tech head tutor of mine was something called the whale-tale- try googling it, bouncing always works well for me

Trying to take any kind of quality portraiture with that horrid pop up flash is absurd to put it mildly. However, simply screwing a bigger flash on top of the camera is not a lot better, it just gives you MORE terrible, harsh light.

You need the flash mounted OFF CAMERA, plus use of some diffusion such as bouncing out of a white umbrella or shooting in a large softbox. You use a remote trigger to fire the flash. I'm not a Nikon person, but I "think" you can fire the remote flash by a very small flash of light from your pop up flash. Read your owner's manual about flash usage and remote triggering.

Pop ups are great if you have a car accident and there's not enough light to photograph that much needed evidence, however in most cases pop ups are useless. Buy the Speedlight!

example… One flash on hot shoe with Gary Fong Lightsphere bounced off white wall to my left

It's only worth it if you will take the time to learn how to use it. Otherwise, it's a waste of money.

I remember I bought my first accessory/external flash thinking my portraits would magically come out better (they did not). Now, 5 years later, I'm finally getting around to making the most of my speedlight flash (dialing down the power, using bounce, etc.). I still have a bit to learn.

I've gotten really nice portraits using the pop-up flash. I'm not saying they've all been good, but not all have been bad. So as you can see, you can't generalize. Nowadays, I'm using my speedlight (in my case speedlite) more and more.

I still use my pop-up flash every once in a while (it works in a pinch). Those who say it's completely useless are going at it all wrong.