The CDA model (Cisco's 3 Layer Hiearchical model) is typically used in a medium to large network - although, it can also be used in a small network, it just might not be financially feasible depending on the company's budget. The 3 layer architecture will also help allocate resources better throughout the network. For instance, the core can do its job and ensure high speed packet delivery, the distribution switches can focus on things like routing between VLANs (or subnets) and QoS, while the access layer can focus on delivering data to the end users. This model also allows much better scalability than the two layer model.
The collapsed core architecture (2 layer) is mostly used in small networks. It may be used for perhaps, a small branch office. It takes the above CDA model and collapses it into 2 layers. Either the core/distribution and access model or the core and distribution/access model. Depending on the size of the network, it might be a wise financial decision to take this route instead of the 3 Layer Architecture. Although, from a technical standpoint, the 3 layer model is the way to go.
Here is a link that may provide some more insight:
http://www.cisco.com/global/AP/switching/deploy_manage.shtml
