إِنَّمَا الْخِلَافُ فِي الصِّيَاغَةِ.
The disagreement is confined to wording.
“Innamā” restricts the location of the disagreement: the disagreement concerns expression, not the reported conclusion. This is a response to a specific objection. It does not assert that no other kind of difference could ever exist in any possible investigation.
مَا الْخِلَافُ إِلَّا فِي الصِّيَاغَةِ.
The disagreement is only in wording.
Negation and exception express a comparable restriction here. In either form, the crucial analysis concerns what is restricted and to what, not merely the name of the grammatical device. The disputed alternative supplies the interpretive focus.