89 gigs of RAM is minimal (just don't keep a lot of apps open and running at the same time), but definitely usable, your processor, by todays standard is relatively slow and 88 GB of storage will work for either Mojave or Catalina. Mojave will offer the opportunity to non-destructively reformat your HD to APFS and Catalina doesn't even ask when you have an SSD, I don't know what it does when installing on an HD. Catalina also moves things around and creates new partitions to further isolate and protect system files from inadvertent damage or malware intrusion but that is largely invisible to most users. I doubt you will see any performance differences between the two.
I will make one final argument in favor of Catalina, it won't become out-of-date for at least six or seven months while Mojave is already behind the power curve. 😎
