Actuallý it seems that Petrucci has had a factory bike for a year and Miller not ..... so the thought is Miller is the better prospect ..... given that they already know what Petrucci can ( or cant ) do.
Plus the small elephant --- compare Miller to Tito for a look at the quality of machinery.
In 2016/2017 they were team mates at MarcVDS where Miller scored 139 points from 30 races competed (he has had a number of DNS and been ruled out of other weekends) whereas Tito has scored 64 points from 35 races starts.
Now, move forward to 2018 where Miller is on 36 points and Tito on 24, each from 4 race starts which kind of shows that the MarcVDS equipment was inhibiting both and holding both back somewhat (IMO).
Petrucci on the other hand has had Pramac equipment (factory supported team on prior years bikes, not full factory since 2015 and has scored 382 points from 50 races across 2015 - 2017 and is currently on 34 points from 4 races).
Nothing really stands out there as one being superior to the other but as mentioned, when you compare Tito and Jack it does give an indication of where MarcVDS were in the pecking order and why they are headed elsewhere