I hope nobody minds me asking, but why are there separate STIX and XITS math fonts, and how can I go about deciding which one I should use? Which one is best suited for which purpose? Is one of the two better than the other?
For a bit of historical context, when the STI Pub released v1.0.0 of the STIX fonts it did not have any math layout support neither for TFM files for traditional TeX engines nor OpenType layout for modern TeX engines. XITS started as a community effort to provide OpenType layout support and make the STIX fonts usable with engines that support OpenType math layout (namely XeTeX, LuaTeX as well as Microsoft Office 2007 and later).
Later STI Pub released a version of STIX fonts with OpenType math layout, but it was incomplete and had known bugs, so XITS remains preferable if one wants to use
A bit later, STIX support files for traditional TeX engines was released (the
stix packages), but XITS does not provide equivalent support, so if one wants to use STIX fonts in a traditional LaTeX setup, the
stix package is the only option (and it works in XeTeX and LuaTeX, too, but using the traditional font machinery of course).
STI Pub is working on next version of STIX fonts that should provide better OpenType math layout and revised glyph shapes (among other things), so it would supersede XITS in certain ways (XITS would still be useful for people who prefer the existing design, or want to do right-to-left math).