The cloak is better for moving around in the open.
The Disillusionment Charm works by mimicking the appearance of background objects and patterns:
Harry looked down at his body, or rather, what had been his body, for it didn’t look anything like his any more. It was not invisible; it had simply taken on the exact colour and texture of the kitchen unit behind him. He seemed to have become a human chameleon.
— Order of the Phoenix, chapter 3
By its nature, this method is most effective when the Disillusioned person stays very close to the background and remains still, so that there are no telltale shadows and no parallax effects due to a changing angle of observation, and so that imperfections in the mimicry are not made more apparent by movement. Harry's cloak does not have this restriction; it renders the wearers completely transparent. As long as wind and aerodynamics (Order of the Phoenix, chapter 3) — and the number of people to conceal — are non-issues, the cloak is the better choice.
Although a relative novice such as Goyle can cast a Disillusionment Charm that will work under ideal conditions (Deathly Hallows, chapter 31), only experts have demonstrated the skill to make them effective under more varied conditions.1
Hermione used Disillusionment (as well as several other charms) to conceal the renegades' camp (Deathly Hallows, chapter 15), but this was out of desperation rather than confidence in the charm's suitability for that situation.
In the book, Harry and Hermione used Polyjuice Potion to appear as Muggles (Deathly Hallows, chapter 16) even while hidden under the cloak. Although Disillusionment underneath the cloak would have made their invisibility coverage slightly more reliable around the ankles, Polyjuice increased their options for staying safe by giving them an entirely different way of blending in. Since the Disillusionment Charm can be slow to remove (Order of the Phoenix, chapter 4) — slower than removing the cloak, at least — it could have hurt their chances of a quick getaway.2
1 Moody, Order of the Phoenix, chapter 3; Dumbledore, in the room of the Mirror of Erised, cited by Harry in Deathly Hallows, chapter 22; Voldemort, Deathly Hallows, chapter 24.
2 It can be done quickly, such as when Moody and the others arrived at the Dursleys' house (Deathly Hallows, chapter 4), but a lot of expert attention and planning had gone into every detail of that mission.