Slug preserves the vector shapes. MSDF first converts to a texture, which can lose small details. How much depends on the size of your texture and the complexity of the font. I expect MSDF will never look better than Slug.
However, MSDF can handle lots of distance-based effects (glow, shadow, outline, gradient, etc.)[1], and I don't think Slug does that. And MSDF is constant time whereas (I think) Slug is going to vary depending on the complexity of the font.
The Slug paper [2] says:
All of the techniques that store data in a texture atlas are inherently using a discrete
sampling of what is actually an infinitely precise description of a glyph outline. This
inescapably leads to limitations that can be mitigated by increasing the resolution of
the texture atlas, but that can never be completely removed. For applications that
need to render a wide range of characters at potentially large font sizes, a texture atlas
capable of producing glyphs at an acceptable level of quality may have prohibitively
large storage requirements.
Slug = Formely patented font rendering algorithm for GPUs, freed 2 weeks ago https://terathon.com/blog/decade-slug.html