Of course they're both valuable. But you cannot reasonably contend that the graffiti is more valuable; it is much less valuable.
> In linguistics for instance graffiti can tell you things about the language and its evolution that the formal language won't communicate, like how certain letters stop being pronounced or become pronounced differently.
This certainly doesn't apply to Latin, which survives today as several robust living languages. We can easily determine how the sounds of Latin changed without making reference to graffiti.
(And of course, the formal literature has quite a bit to say on the sounds of the language, too.)
One reason Pompeii graffiti is valuable, is that it provides some absolute dating of Vulgar Latin features, while the modern Romance languages show only the final outcome without those exact dates.
I think you underestimate just how grateful historical linguists are for popular texts beyond literary ones. Another example is the Novgorod birchbark letters, which not only shed light on old Russian but some of their features (found in no other source) required scholars to completely revise their reconstruction of Slavic historical phonology in general.
> In linguistics for instance graffiti can tell you things about the language and its evolution that the formal language won't communicate, like how certain letters stop being pronounced or become pronounced differently.
This certainly doesn't apply to Latin, which survives today as several robust living languages. We can easily determine how the sounds of Latin changed without making reference to graffiti.
(And of course, the formal literature has quite a bit to say on the sounds of the language, too.)