Versus est un mot d’origine latine que j’ai toujours vu être utilisé aussi bien en anglais (très courant) qu’en français (moins courant). Je précise que cette fréquence d’utilisation est celle que j’ai notée au fil du temps, elle n’a rien de “scientifique”.

D’ailleurs, il semblerait que l’Académie Française désapprouve l’emploi de “versus” mais je n’ai pas trouvé sa recommandation pour une équivalence également raccourcie. Par contre, dans des petits guides à usage de l’administration, on trouve la proposition “vs” (lien) lorsqu’il s’agit de prendre des notes.

Afin de comparer ou d’opposer deux valeurs, j’ai déjà vu également les signes “/” et “//” dans le sens de “par rapport à” puisqu’on met les deux valeurs en parallèle.

Dans le cas d’un bouton d’application, il me semble que l’abréviation “vs” est appropriée car la plus compréhensible et concise. Je ne lui trouve pas d’équivalent aussi parlant, transcrivant de manière simple l’idée de départ.

French borrowed versus (vs) from English in 1965 and is nowadays widely used in the academic world. It is in all recent dictionaries, e.g. Larousse or the Dictionnaire historique en langue française (not online).
The Académie Française discourages its use because French has
“contre, ou, face à, en face de and par opposition that can express the same idea and therefore it is not necessary to use this Anglicism.”¹

But the Académie Française is considered an old stilted institution by a lot of people, included academics who use vs quite profusely in their papers.

An example in this paper (p.58)

même si elles semblent converger vers quelques valeurs-nœuds, celles, par exemple, de l’être vs l’avoir ou de la participation esthésique (et esthétique) au monde vs des vérités débrayées ²

And this other one (two occurrences). Quite easy to find plenty more.

I am not sure though that it would be understood by all French natives, but no one knows all the words in their mother tongue if they are not part of their everyday conversation. I rarely use apps so I do not know if vs is already used for what you describe (s.o. on this site will surely do) but I would not worry about leaving vs as is. I am pretty sure you would not find anything as short and saying just what you mean.

Edit
@LaurentS pointing out in a comment that versus has long been used in video games that leads to add that it has reached other strata of society than academia, and is much more popular than I thought and would be perfectly fine for an app. And probably better understood than the abbreviation c. (for the French contre) which apparently exists but I have never seen used.

To answer the comment about italics I point out that academic papers have specific rules about using italics and other formatting but versus is rarely in italics outside academia.

ces affaires vont affaiblir «durablement l’ensemble des syndicats de salariés en France», versus 30 % qui parient sur un tassement. (Le Figaro, 18/11/2018)

Un site français d’évaluation de téléphone mobile.

la coronarographie normale ou avec lésion modérée est deux fois plus fréquente chez les femmes que chez les hommes (12 % versus 6%) (Le coeur des femmes : comprendre les maladies cardio-vasculaires– 2013).

À Paris : quartiers huppés versus quartier latin (L’adolescente et le cinéma de Lolita à Twilight – 2013)

---

¹My translation.
² My emphasis.

Since the context is a toggle button in a graphical user interface, I assume your priority is less to find the exact official (approved by the Académie Française) equivalent to “vs”, than to find something bearing approximately the same meaning with the least number of letters.

vs would be acceptable but this is not widely used and, depending on your audience, some would not understand its meaning, especially uneducated people.

diff and comp (or just cmp) are abbreviations (respectively for “différence” and “comparaison” / “comparer”) that may have a better chance of being understood. However, none of those words is a translation of “versus”; they just match the action associated with your button.

If a four letter abbreviation is still too large for your button, you may also look towards Unicode symbols like those double arrows: ⬄, ⇎, ↭, ⇋, ⇄, ⭾, etc. Or just ±. Be creative!

To highlight @Patrick T’s note: Depending on the context, a simple « ou » could do the job.