I think there are more effects in place. I agree that it is probably better to onboard everyone.
But if I should choose what to focus on I would prefer to make first merchant more solid. So that he sees the bitcoin in action. He can than help with onboarding others and become useful bitcoin beacon - for example for p2p buy and sell.
In another words I think that (at least in my neighborhood) we need now more people paying with bitcoin than merchants who accepts it.