I will say that the technical difference between the two images is that the first is a continuous single exposure and the second is an amalgamation of separate smaller exposures. All the advice in books and the internet warn against very long exposures on digital cameras and although I knew why, it's always good to see for yourself. The difference in image noise between the two pictures is startling; even at this scale the noise in the first image leaps out and ultimately makes it very unsuccessful. If the noise were reduced to the level of the second image, you might even have been able to resolve the Milky Way which was prominently visible that night, and oh so many more stars! This on its own has been enough to convince me not to try such a long exposure again, to say nothing of the immensely long processing time and the lack of control when it came to oversaturated portions of the image.
I have declared to myself that I shall produce a good star trail image by the end of the Summer. We'll see.