I just spent a lovely week on Sardinia. The recently split Mediterranean Flycatcher (Muscicapa tyrrhenica tyrrhenica) is pleasantly common there.
It seems to occur in a much wider range of habitats than Spotted Fly in NW Europe. From macchia to hill forests and from quiet farmland with scattered trees to village gardens: they seemed to be everywhere below the tree line.
They differ from Spotted on plumage, biometrics and genetics (see Viganò and Corso 2015). I indeed had the impression that on average they were paler and less spotted below, but note that some were more marked than others, while on the other hand less well-marked Spotted Flycatchers can be found in NW Europe.
For now I reckon we are still a long way off to identify a potential vagrant in NW Europe with certainty.
The "Spanish" Med island ssp. balearica seems to differ a tod more from Spotted, with a pale speckled crown. Check out Marc Illa's wonderful website for more details and an instructive pic.
The birds on Sardinia were very vocal:
A quick comparison with five recordings of Spotted Fly on xeno-canto slightly suggests that the calls I recorded are on average slightly higher pitched, though I have no clue about individual variation (and with high pitched calls the quality of the recordings certainly plays a role), so it might be nothing at all.
My recording of the alarm call is rather poor, but I couldn't find any obvious differences. I never heard its song.
Word is on the street that Viganò and Corso are about to finish a new piece on this taxon that includes calls. Something interesting to look forward to, cause I hope to learn a bit more about these birds!