Co-located Topology 

Requires fewer machines. Because the HADB nodes and the Application Server instances are on the same tier, you are able to create an Application Server instance on each spare node to handle additional load. 

Improved CPU utilization. Processing is distributed evenly between an Application Server instance and an HADB node sharing one machine. 

Useful for large, Symmetric Multiprocessing (SMP) machines since it takes full advantage of their processing power. 

Increased complexity of maintenance. For example, when you have to shut down machines hosting HADB nodes to perform maintenance, application server instances on the machine also become unavailable. 

Separate Tier Topology 

Easier maintenance. For example, you are able to perform maintenance on the machines that host Application Server instances without having to bring down HADB nodes. 

Useful with different types of machines. You are able to allocate a different set of machines to the Application Server tier and the HADB tier. For example, you are able to use more powerful machines for the Application Server tier and the less powerful machines for the HADB tier. 

Requires more machines than the co-located topology. Because application server instances and HADB nodes are located on separate tiers, application server instances cannot be located on the machines that host the HADB spare nodes. 

Reduced CPU utilization. The application server tier and the HADB tier will likely have uneven loads. This is more significant with a small number of machines (four to six).