Both of these are good and vital to a growing church.
Informal ministry is simple good gospel deeds; they go under the radar, usually only seen by one or two. It’s things like prayer, meals, phone calls, catching up, helping, sharing life together.
Formal ministry is organised and/or appointed; it’s running the youth group, leading the growthgroup, putting a plan together for welcoming new people, organising the camp away. These people bear a weight of responsibility appointed by others (cf Acts 6).
If you cut all the formal ministry, and only had informal ministry… you might appear to be a loving community… but how would you know if everyone is getting informally loved? You’d need to appoint someone to be responsible for that! You need leaders, you need organisers, you need these roles in order to love many people.
If you push everyone into formal ministry only, people won’t have time for the messy business of life and helping people in the unplanned hard bits of life. You’ll miss out on loving people really well.
So surely we should endorse both… we should encourage people to look for ways to express Jesus’ love informally, take initiative and love others without asking for permission. And we should encourage people to work out how to serve “the body” with their gifts from God in formal ways, for the sake of many.