In the case of Vikings, what was the advantage to them? Their paganism made them fearless warriors, they were not afraid to die because if they died in combat it was a glory to them. They'd meet the Gods in Valhalla. Later they adopted Protestantism which further proves that the idea of a punitive God didn't fit their culture.
About romans, do you think adopting Christianism was a political decision made by the Emperor to instill fear (of hell) in the populations and better be able to control them?
The way I see it paganism gives more power to the individual, while Christianism, by instilling the concept of heaven vs. hell tries to control people through fear: if you don't do as we tell you when you die you will go to this awful place, while if you do everything you will go to heaven.
So while in the Romans it made sense to apply Christianity as a weapon of control of masses, in case of Vikings it didn't...
I've answered this question a couple of times on a broader scale, most recently here. Most of the points apply to the Norse as much as to any other early medieval polity. It's slightly odd that you ascribe a level of personal freedom to Paganism when we have almost no idea whatsoever of actual Norse Pagan religious practices.
To say that their strain of Paganism made the Norse fearless warriors can imply quite incorrectly that Christianity doesn't have a similar martial outlook and concept of an eternal reward for martyrdom, a perspective that the sheer existence of the Crusades should readily dispell. Long before this, however, Christianity had a long history of 'soldier saints', from the more passive Roman soldiers martyred during the Dioclesian persecution, to the more aggressive Anglo-Saxon saints such as the 7th century Saint Edwin, who fell in battle against the Pagans. Asser's Vitæ Ælfredi paints Alfred's campaigns against the Vikings as a struggle by brave Christian warriors to defend their homes, and by extension all of Christendom, against a Pagan threat. After Alfred's decisive victory at Eddington, Guđrum, and by extension his army, converts to Christianity. Although this was most likely at spear-point, there is an implication, at least by Asser, that Guđrum accepts so readily because the completeness of the Christian victory tells him that their cause is more just and their god more powerful, and it is therefore in his best interests to convert.
Beyond this narrative, the standard reasons for conversion - particularly the bureaucracy and political control - still stand.