SB-400

• low price
• so small you can leave it on the camera all the time, or put in your pocket
• simple to use

SB-600

• can use off-camera
• head tilts and swivels, also zooms to get max power at greater distances
• more complicated to use
• large and heavy

I own both. I mainly use the SB-600 off camera and will use the SB-400 for family stuff. Unless you think you may use it off camera, the SB-400 is a great little flash for half the price. It's true you can't swivel it, so in portrait orientation you'd have to bounce off a wall, rather than the ceiling, or not bounce at all. For general photography I don't find the lack of swivel to be that big a deal. If you're taking family snapshots, I'd lean towards the SB-400.

SB-400

• low price
• so small you can leave it on the camera all the time, or put in your pocket
• simple to use

SB-600

• can use off-camera
• head tilts and swivels, also zooms to get max power at greater distances
• more complicated to use
• large and heavy

I own both. I mainly use the SB-600 off camera and will use the SB-400 for family stuff. Unless you think you may use it off camera, the SB-400 is a great little flash for half the price.

It's true you can't swivel it, so in portrait orientation you'd have to bounce off a wall, rather than the ceiling, or not bounce at all. For general photography I don't find the lack of swivel to be that big a deal.

If you're taking family snapshots, I'd lean towards the SB-400. It is considerably better than the pop-up flash, and it's far more convenient and easier to use than the SB-600.

If you think you may want to try off-camera flash, or you think the lack of tilt/swivel is important to you then go with the SB-600.