Probably musical forms. There may also be a difference amounting to what, in Anglicanism (where Methodism came from and is still a variety of though not in ecclesial unity), is called low church and high church services. High church would be more Anglo-catholic; low church would be, um, more Baptist for want of a better description.
I kind of doubt that’ll be the distinction – I imagine it’s more about the musical type and maybe how formal the language is – because I’ve never heard of a congregation giving both low and high services and it seems like something a little too important to vary on. But theoretically I suppose a single church could do that to accommodate different members. Come to think of it, I’ve personally (through my family) heard of at least one Roman Catholic congregation who did that; Mom and Dad went to what Anglicans would call the low service, but it also featured ‘high’ services. And now I recall C. S. Lewis maybe relating that an Anglican congregation did that and he preferred the low service. (But I might be reading too much into what he said about it.)
On the point about trinitarian or non-trintarian: while a church could have guest members, so to speak (regular attenders who help contribute somewhat to the upkeep of the church), I do think there comes a point where both the leadership and the membership (per se) of the congregation has to take definite doctrinal stands on what they agree to believe to be true about the deity they’re religiously worshiping. Certainly there are risks in doing that, of people thus coming to take merely tribal identification and competition stances; or even worse taking an effectively gnostic stance of salvation by doctrinal passcard assent. But a concern for truth means believing what is seen, so far as possible, to be true, and not believing what is seen, so far as possible, to be false. Maybe it would be better to say, for such purposes, that analogically trinitarians and modalists and unitarians of various sorts (and binitarians if any such exist) are cousins in Christ, though we can also be friends and in many ways allies under Christ. But as the ideas about God Himself get more and more different, the family relation (analogically speaking) also has to get quite different. There are major conceptual differences about how God operates between even members of the Big Three Theisms on one hand, and nominal deists on the other – and the deists are still supernaturalistic monotheists! The conceptual differences shatter even harder between supernaturalistic monotheists on one hand, and polytheists, or cosmological dualists (or other high-ontology multi-theists), or pantheists; and more still between them and atheists; and ALL THOSE GROUPS could (and sometimes do) easily have various levels of high regard for Jesus of Nazareth personally. But they are not at all saying the same kinds of things, ultimately, about Jesus (or the Holy Spirit either), much less the same things ultimately about God.
So yes, I think it does make a difference who counts as being in a group and why. Am I supposed to religiously worship and trust myself for salvation? No. (And what does “salvation” even mean, and to what degrees?) Am I supposed to religiously worship other humans and trust them for my salvation? No. Am I supposed to religiously worship any lesser lords or gods and trust them for my salvation? No. Am I supposed to worship Jesus (the Son), the Father, and the Holy Spirit, and make disciples in their single name? Yes. Am I supposed to trust them for my salvation? Yes, and not any lesser lord or god. Are they in real interpersonal relationships with each other? Yes, not illusionary ones. Are there multiple Gods Most High? No. What does it mean to be God Most High? To be the one and only ground of all reality. Like in atheism? No, the fundamental ground of reality is intentionally and rationally active. That also means coherent and mutually supporting interpersonal relationships are also the ground of all reality, and that puts morality and reason both at the foundation of reality – which makes a huge difference in how I ought to regard any not-God portions of reality, too, and which makes a huge difference in how I ought to regard salvation.
These topics, and the other variant answers to them, have not in the best cases been whipped up out of nothing for no better reason than for personal advancement and emotional gain even at the expense of other people in various ways – although even those answers to those topics have often been abused for such reasons. But the various answers to those topics do make categorical differences, in various ways, about how we ought to be dealing with reality; and if a group isn’t about cooperating together to deal with reality then the group isn’t even really a group! (For some pantheists, and for some types of atheist, a group literally cannot be really a group anyway. For other pantheists, a group ultimately should not be a group, the group-iness itself being endemic to what’s wrong to be saved from.)