I must state, I am a little bit disappointed that Professor MacCulloch didn’t really seem to answer this question, however, I think some of his points were great. For instance, would the Arians have enacted the bureaucratic inquisition as did the Catholics or would they have taken no legal action against purging heresies? Based on an essay that Sir Isaac Newton wrote, I would say that the Arians would have in fact permitted Trinitarian Christians in their presence to worship with them. But alas, we’ll never know.
A really interesting point that MacCulloch points out is how the Eastern Orthodox are more mystical than the Catholics but have no rationale or form of argumentation for their belief systems as do the Catholics. In other words, where the Catholics lack strength, the Orthodox possess it and vice versa affirming the position that Catholic and Orthodox Christians need each other in order to thrive.
And I would definitely agree with the last point that MacCulloch makes–the Reformation was the Arian’s last act of revenge–now they’ve won in the form of the Jehovah’s Witnesses!