Christianity is one religion. Christians in Spain, Italy, and France would have rivalries and fight wars with one another but they would also unite against non-Christian enemies (a la Crusades). If there were non-Christian states in close proximity they would probably be even more united. Paganism on the other hand is not a single religion. Norsemen may not feel any more kinship with Pagan Celts then they do with Christian Franks. (I think this is debatable. I mentioned it earlier)
The Crusades are a pretty tepid example of Christian unity even at their best. To paraphrase someone from (admittedly) the 16th century, enough Christian strength lies in the grave from fights between Christians to fight a crusade against the Turks.
And this was the normal state of affairs. Hell even in Iberia, you see the Christian kingdoms bickering with each other as eagerly as carrying out the Reconquestia.
There may be times when Christian nations would go to war and each ally with different pagan nations against each other. But at the end of the day, they would still acknowledge other Christian nations above others. What I am trying to say is that I am unsure if Pagan nations would have the same sort of implied kinship with each other since "Pagan" is not a religion. A unified Christian world has a better shot at developing wealth and projecting its influence than a series of independent isolated Pagan states.
Except that there isn't a united Christian world any more than there was OTL, which is to say not in the least.
(If it isn't already abundantly clear I am not implying that Christianity as a religion is inherently better equipped for empire building than any other religion. If "Pagan Europe" was united under a single Pagan religion my argument completely goes out the window. Hence why I mentioned the possibility of a united Scandinavian-dominated Northern Europe.)
The only way Christian Europe would in any way shape or form be better off would be if either pagans were even more belligerent (difficult) than OTL Christians or if Christianity was superior.
United in a single religion is not unity in peace at any point between AD ~500 and the Reformation. And even within the Catholic world specifically for example, France-Habsburgs.
The idea that one religion means anything here just doesn't stand up to scrutiny.