Mini,
I usually suggest to my users that custom form fields are for items that need to be quickly referred to by everyone or reported on (tracked as a metric) in some way.
To us, documents are then the opposite of custom fields. They are useful as a container for historical data that doesn't have to be quickly referred to by the majority, or reported on/tracked as a metric. They also operate as a way to display chunkier content (think essays, or large spreadsheets).
Updates for us, are only useful for quick "in the moment" news and as a historical timeline of all "in the moment" types of news (if you look at the updates area of a project you will see that it contains updates from all tasks, which makes it really unwieldy to wade through). Again, updates are not for chunky content -- there's definitely a limit on how much text you can expect people to read since you can't really format an update yet.
If your final decision is of the type "yes/no" and you want to report on it, then put it as a custom form. If the decision only had to be referred to at one moment in time, it could live in an update on the project and get quickly lost at the bottom of the heap. If it's a lengthy essay, you might find it's best if it exists as a document--especially if it only has to be reviewed by the PM one time and never again.