It is a matter of taste and circumstances of the campaign. It is the same amount of work to create a campaign as a linked series of adventures, as it would to offer complete freedom of choice.
Since the freeform campaign is less commonly known a detailed explanation is needed.
The free form nature of role-playing games are difficult to manage. It is understandable why many choose to run their campaigns as a linear series of adventures. To allow for more freedom you need to get away from the idea that a campaign is a connected series of adventures.
To do this you need to develop a “Bag of Stuff.” The elements inside your “Bag of Stuff” are pulled out and combined during a session to form the adventure the players are experiencing. A campaign cease to be about prepping adventures but rather about managing and expanding what in your “Bag of Stuff”. The referee creativity becomes focused on judging the consequences of the player’s action.
What goes into a “Bag of Stuff? The broad categories are Items, NPCs, and Locales.
Items are the physical object found in the setting both mundane and supernatural.
NPCs are the characters including generic template that can be customized on the fly with a name and personality (Barkeep, guard, etc).
Locales are descriptions of sites both specific and generic. This part is the most like writing an adventure except the effort should be focused on description not plot. A Palace could be a setting for an audience with the king one session or the scene of a raid on the royal treasury the next. General locales are generally the most flexible. A typical church of the god of honor, a peasant hut, a manorhouse.
The more well-read the referee is the more able they will be able to customize the generic elements into the specific items the PCs encounter.
Coupled with this is a “World in Motion”. Making a living breathing setting that exists outside of the player’s actions. To prepare the “World in Motion” for play, the referee draws up a timeline of what going to happen in the setting for the expected length of the campaign. It may be a year, two years, or a decade. This timeline is written has if the characters did not exist. It will guide the referee as to what specific items, NPCs, and Locales need to be added to the “Bag of Stuff”.
The “World in Motion” comes into play through the background color, news, and rumors the referee uses during play. Referee will focus a timeline on events that are of interest to himself and his players.
Managing the campaign is about deciding the consequence of player actions and their effect on future events. The referee will need to be prepared for drastic alterations if circumstances required it. Above all remember that the timeline is a plan not a script. Like a plan of battle it changes once put into action.
Also understand that the initial starting circumstances of the characters will dictate how linear subsequent actions are. A campaign involving the characters as Royal Guards undertaking missions for the king is considerably more linear than when the characters are free-booters roaming the countryside. Both allow players complete freedom of choice but one results in a more linear flow of events than the other. But even with the restricted setup of being Royal Guards that campaign could have considerably variations as the players guide their character through their lives as guardsmen of the king.
The creativity of the referee comes primarily in deciding the consequences of the player’s actions. Not just picking out the likely consequences but the one that are both probable and interesting This is because we are playing a game not writing a alternate history thesis.
Using the “Bag of Stuff” and the “World in Motion” as tools in managing a campaign referees will find that they can allow players to have considerable freedom within the setting and the amount of prep work remains the same as a campaign comprised as a series of linked adventures. If the setting is retained for subsequent campaigns you the prep work becomes considerably reduced as much of the material is recycled into the new campaign’s “Bag of Stuff”.