What are the differences between these two Grids? And why use Animation Node instead of the array modifier?
When using the Object Instancer node, you are basically duplicating the object while keeping the same mesh data block. When using the Replicate Mesh node, you are constructing a single mesh by replicating the source mesh along the input transformations.
The array modifier would be roughly equivalent to the replication method, however, it is very limited when it comes to the patterns that you can generate.