What exactly were the technical advantages that made their Unix worth more than Xenix?
For most parts: The Name. Otherwise it's simply the next release of SCO's unixoide OS.
They were only sold in parallel for a short time (ca 1989/90). While the latest Xenix version was based on System V R2.3, SCO Unix started out as System V R3.2. But using the same driver interface as Xenix.
Xenix had a long history of development since ca. 1980, but when SCO acquired the rights to the Unix name, it was clear that an 'all new' product was to be released :))
Originally Xenix was developed by Microsoft. MS licenzed Unix V7 already in the late 1970s. They did, at that time, firmly believe that Unix would be the future OS for all of their products (they were a language company) - and most of the market. They did license the source code, but not the name, so they came up with Xenix as their brand name for Unix.
MS did not sell Xenix to end customers, they only OEMed it to other companies. Much the same way they sold MS-DOS only to OEM before DOS 3.2. Notable licencees were Intel, IBM, Tandy, Siemens (Sinix) or, well, SCO.
Xenix developed from being a System 7 over System III to System V related. Unlike often assumed, Unix does not need a fully fledged MMU, it only simplifies life. In fact, SCO's twist on Xenix was the low end market. Their Xenix 3.0 (System III alike at that time) was intended for IBM-PC (MS-Xenix did of course use 286 protected mode). Not a rocket, more a soap box racer, still, worked quite fine and covered a market other unixoid systems didn't.
It wasn't until the mid 1980's (85?) that SCO Xenix was as well available for 286 class machines - now System V alike. At a later point 386 got native support as well.
In the late 80s (1987?) MS sold all rights, code and customers of Xenix to SCO for a considerable stake in SCO. Now SCO did all development on their own.
Again later (1989?) SCO bought the right to the Unix name. After that the next major Xenix release was of course renamed SCO Unix. SCO Xenix was, under that name, still maintained for 3 years for existing customers - like other releases before - but otherwise the name was history.
Other MS licencees, like Siemens, continued their development independent way into the 90s and 2000s.