Kind of a toss up. Doordash has more market share in US but UberEats has wider international presence and is a leader in some markets. But yes, Uber acquired Postmates at one point and is now rolling out a subscription program called UberOne that encompasses rides, eats and its nascent grocery/pharmacy offering from the cornershop acquisition.
Also depends on the metro. I think NYC is actually primarily GrubHub, Seattle and central TX are primarily UEats, but for the most part DD is a safe bet. North TX is definitely DD territory.