This is a bit of a morbid comment here, but here is my understanding of this choice. A serrated knife will indeed cut rope and seatbelts much better. As far a living things go - when the body had a jagged cut it knows to automatically seal up that vein or artery. A serrated knife causes this kind of cut. A plain blade causes a much more surgical cut, in turn the body does not readily recognize it and the blood keeps pumping out. Again, this is a morbid commentary, but if I do indeed have to cut someone it means they are actively trying to kill me and I can not run away. In that case I want them to bleed out quickly. I prefer a plain edge for this reason. Additionally, in some areas a serrated knife is illegal and a plain knife seldom is. Whatever you choose I hope you never have to use it for defense. Knives just plain scare the crap right out of me. I would much rather have someone shooting at me than swinging a knife at me. Good luck.
Jake