Sb400 or 600 for speedlight upgrade?

Get the SB600. It is compatible with modern Nikon film SLRs, and will work in TTL or iTTL. It is heavier and bigger than the SB400, but has more versatility and power. It will also function off camera as a slave if you upgrade your body in the future.

While the SB 400 will work on your camera, it's actual intended use is as a slaved fill flash for the Nikon CLS (Creative lighting system). The SB 600 is a much better flash to work with in this regard. Some camera bodies have a "Commander" mode built in so you can remotely control the SB 600, 800 or 900 models, so you could look for this upgrade.

The other option is to buy a SU-4 wireless control and the SB 600. The SU-4 mounts to your hotshoe instead of the flash and will allow you to remotely control the SB 600. You can take these components to the next camera body you buy rather than having to replace the system if you upgrade your camera body.

To piggy back on foto4zFX answer, without understanding the type of digicam you have solutions is basically no longer as precise as they might. some Nikon dSLRs have not got a "commander mode" which permits the digicam to set off an off digicam flash. if it is the case, look on your digicam handbook and notice if that's a threat. If no longer, then the digicam will basically use the in-built flash(if it has one). nonetheless, the pop up flash might attempt and turn on in case you have an SB-600 related, yet no longer powered on and are shooting in automobile mode. If the gentle is such that that's basically too low or backlit, the digicam will attempt and use the single digicam flash. because of the fact the SB-600 power isn't on, it would not understand that's connected and could attempt and use the pop up flash. in case you're relatively in aperture priority mode, that's "A" on the command dial, then the flash shouldn't pop up till you press the button on the component to the digicam physique that forces the flash to pop up.