There are two main criteria that determine how we will finger a passage in (or to) thumbposition: technical (“which fingering is the most secure?”) and stylistic (“which is the most appropriate?”)
The difference in style (aesthetics) between the Romantic Period‘s tendency to swoop and soar, and on the other hand, the Classical Period‘s discretion and “cleanliness” (see Pedagogical Order By Epoch In Thumb Position), has profound fingering implications for the cellist. We can finger passages “Romantically” (staying on the same string) or “Classically” (playing “across” the strings).
In “Classical Style”, this Beethoven example could be played across the strings (as indicated in the suggested fingerings). This “stay- in-one-position” fingering not only makes the passage infinitely easier but also avoids the use of stylistically-inappropriate swooping romantic glissandi. In spite of the shifting difficulties, most cellists however choose to play it on the A-string for the more soloistic sound quality. And, what’s more, this is Beethoven’s Opus 56: we are now coming into the Romantic Era so perhaps the “classical” fingering is already outdated (or almost). The Tosca theme is a perfect example of a soaring Romantic melody that desperately wants (and needs) warm vocal glissandi to accompany the large melodic leaps. Fingering it “across the strings” in such a way as to eliminate these gorgeous glissandi, as though it were a charming classical-style melody would be completely out of style.
See also the articles on Technical Fingerings and Thumb Horizontal String Crossings.