OK, here's the long answer to your post:
For a lot of information regarding the difference between UD I's and UD II's, I would refer anyone who is interested to both the old dcc discussion group (http://www.dcccompactclassics.com) and the MFSL FAQ at: http://www.aurealm.com/zinc.htm#both%20UltraDiscs%20and%20UltraDisc%20II.
As indicated in some of the above posts, all UD I MFSLs were pressed in Japan (except for UDCD 503 and 563), whereas the UD II's were pressed in the United States. Approximately 40 discs were released in both UD I and UD II versions (UDCD 505, 507, and ~509-567, with some exceptions).
As stated in the MFSL FAQ, the reason MFSL originally went with a Japanese CD pressing plant is most likely because at that time (1987) there were only a few US CD pressing plants, all of which were running at full, or very close to full, capacity, and were thus unable to guarantee meeting either Mobile Fidelity’s quality standards or pressing needs. In addition, at that time it was commonly believed that the quality standards of Japanese plants were the highest of any in the world.
According to a few individuals who have compared the UD I of DSOTM (and other MFSLs) to their corresponding UD II versions, the UD I version sounds superior, allegedly on account of the higher quality of pressing available in Japan compared to the U.S. at the time. (I only have the UD I of DSOTM, so cannot comment). In any event, it is undisputed that the UD I version is more rare.
Thus, if you can get the UD I for the same price as the UD II, go for the UD I.