I love [name_m]Mateo[/name_m]. I do prefer the [name_m]Matteo[/name_m] spelling though. My kiddo has two [name_m]Matteo[/name_m]'s in her grade (of 44 kids). I live in a predominately Hispanic area where Spanish is either everyone’s first or second language (it seems like!) and I’ve yet to see anybody spell it [name_m]Mateo[/name_m]. Either way, it is a gorgeous name!
I really like [name_m]Mateo[/name_m]/[name_m]Matteo[/name_m]. For me, the spelling would be a clue to the pronunciation: [name_m]Mateo[/name_m] is Mah-[name_u]TAY[/name_u]-oh, & [name_m]Matteo[/name_m] is MAH-tee-oh. However, I don’t speak either Italian or Spanish, so I don’t know if the two different spellings actually have different pronunciations or if either of the pronunciations I wrote are the correct one. In any case, it does work really well w/[name_m]Emrys[/name_m]. Good luck! 
[name_m]Mateo[/name_m] sounds really hip but not yet trendy.
My only reservation might be in constantly repeating it or teaching people to say it. I just hear someone saying Matio like patio…
Since [name_m]Matteo[/name_m] is a form of [name_m]Matthew[/name_m], it makes sense that you would stick with [name_m]Matteo[/name_m] and not [name_m]Mateo[/name_m]… which looks a bit like tomato.
I guess some people spell [name_m]Matthew[/name_m] with one T, but [name_m]Mathew[/name_m] doesn’t look quite right.
I like [name_m]Matteo[/name_m] and also [name_f]Mattea[/name_f] (the girl form). [name_m]Matthew[/name_m] has become extremely common, but [name_m]Matteo[/name_m] is a very good choice!
I [name_u]LOVE[/name_u] it 
I like the [name_m]Matteo[/name_m] spelling as well, but either way I think it’s a great name