"Additive" can lead to a mess - if there's any brightness or noise in the dark areas it can accumulate and overpower the stars.
"Bright" is usually the way to go, you can set your exposure length to work with any other features you've included and let your interval composite run as long as you like to get the trail effect you're after..
You can always shoot separate images using interval mode (disable the in-camera composite), and combine on your computer later. A free program like "StarStax" can let you try out the different blending options and see how they work on the same set of images (obviously the software has more blending options, but it includes the camera ones). It can be instructive and help you understand the in-camera options a little better
.