Please note that all I’m trying to point out here is the difference in stable memory usage by these two configurations. It should also be noted that the main cause of these differences is (for most people) going to be how PHP is spawned and handled by the two configurations.

Apache on the DreamHost configuration will (by default) use FastCGI for PHP, this will spawn processes as needed (and end them over time) which can lead to large spikes in memory usage. Predicting memory usage in this setup is difficult, to say the least.

Memory Usage of DH's Apache Configuration

Nginx on the DreamHost configuration is a bit easier on the eyes which you can see from the graph below. It should be noted that I run several more sites on my Nginx configuration then I do on the Apache configuration and because of this I run a much larger pool of PHP processes which increases the overall memory. If both servers were running the same set of sites with the only difference in configuration and applications being the selection of Nginx instead of Apache, memory usage would be lower and still predictable.

Memory Usage of DH's Nginx Configuration