Others will offer a more knowledgeable response than I ... but lack of knowledge has never stopped me from plunging ahead, so here's my $0.02:
The advantage of culturing your own will be the rich complexity of bacteria and therefore of flavors.
The disadvantage of culturing your own will be the rich complexity of bacteria and therefore of flavors ... some of which may not be so desirable, and any of which may be less than predictable.
Since you have fresh / raw milk to work with, you're going to get some good complexity of flavor regardless. Thus, if it were me, I'd use the prepared and predictable cultures to set the dominant flavor(s), and let the richness of the raw milk provide the subtleties.