I often have a hard time deciding whether to use a CCK field or the taxonomy module when building Drupal sites. At Drupalcon, I was glad to see that I am not alone in my confusion. At the Drupalcon Drupal Taxonomy Revisted session, I finally started to understand the use case for each.

Here’s a table I put together to help me quickly decide whether to use taxonomy or CCK in the future.

Question
Which to Use?
Rationale
Do you have a lot of terms or attributes?
As the size of your vocabulary or field option list grows, taxonomy becomes a better choice
CCK stores its values as text while taxonomy stores values with an ID. This makes lookups over a large data set more efficient for taxonomy.
Are you using a hierarchy?
Taxonomy
Taxonomy is designed to handle multiple levels of hierarchy, which a CCK field is not
Are you listing attributes? For example, listing the color options of a car.
CCK
CCK makes more sense if you have created a content type and are adding attributes to it using CCK fields. For example, I could have a car node type and color would be a CCK field
Does your piece of content exist in the real world on its own?
Taxonomy
Taxonomy terms are designed to map to real-world objects in a way that CCK fields are not. For example, if I have a taxonomy of US States, each state exists whether or not I have assigned content to it.
Will you re-use your term or attribute in multiple ways within your site or re-organize them?
Taxonomy
If you have a classification such as US States that you plan to use in multiple node types, then taxonomy makes more sense. It is easier to add new terms to taxonomies over time than to individual CCK fields.
Are you using CCK or taxonomy to list critical information on your site? What is the cost of losing the data if you upgrade to a future major Drupal version?
Taxonomy
Taxonomy is more futureproof because each term has its own unique id, and, it can be moved between vocabularies. Taxonomy is also a Drupal core module and there will almost certainly be an easy upgrade path from Drupal 6 taxonomy to Drupal 7 taxonomy.
An Example: Classifying Topic Areas

Using this criteria, making the decisions I was stuck on in an earlier blog post becomes a lot easier.

Question
Response
Decision
Do you have a lot of terms or attributes?
Yes, we have at least 200 terms and that number is likely to grow dramatically over the next few years.
Taxonomy
Are you using a hierarchy?
Yes, we have at least 3 levels of hierarchy.
Taxonomy
Are you listing attributes? For example, listing the color options of a car.
No, we are listing topics that span multiple content types.
Taxonomy
Does your piece of content exist in the real world on its own?
Somewhat. Greenhouse Gas Inventories exist in the real-world, as does the “Climate.” The category “Campus Operations” is more of a grey area
Probably Taxonomy
Will you re-use your term or attribute in multiple ways within your site or re-organize them?
Yes, we plan to classify many different node types with the same terms
Taxonomy
Are you using CCK or taxonomy to list critical information on your site? What is the cost of losing the data if you upgrade to a future major Drupal version?
It would be painful to lose all of our classification information but I don’t think that it would destroy us as an organization.
CCK or Taxonomy

After evaluating which to use with my new handy chart, it’s clear the taxonomy makes more sense for classifying our topical areas.