I previously held the former view, but having reconstructed Marcion's Apostolos, I learned the following which made me change my mind to a position that aligns now better with traditional scholarship on the beginnings of gospel-writing (as, for example, the two-sources hypothesis). Here are the reasons:
In *Gal 5,21 we read: "I warn you, as I did before, that those who live like this will not inherit the kingdom of God."
This is a back-reference to which the only place to which this reference can refer is *1Cor 15:50: " I declare to you, brothers and sisters, that flesh and blood cannot inherit the kingdom of God, nor does the perishable inherit the imperishable."
From this I have learned that the back-reference in the *Apostolos has escaped Marcion's redactional wit and refers into the wrong direction. This is only possible, if he made use of an earlier collection of letters where 1Cor stood before Gal.
As Marcion in his *Apostolos makes also use of a 3-letter-collection of *Deuteropaulines, it shows that his redaction of the *Apostolos is entirely based on collections. If so, it is most likely, that the same redactor also made use of collections when putting together his gospel.
