Oracle 10g Grid is a concept whereby multiple servers (nodes) can be connected together to meet your needs. Some nodes can run your database. Other nodes can run your application (web) server. You can add nodes on the fly. You can shift a node from one role to another on the fly. Basically, that is the concept of "grid" computing.
To run Oracle's database in a grid environment, you must add on an additional product called Real Application Clusters (RAC). RAC lets multiple nodes connect to the same database. So as you can see, there is no way to determine which is better, RAC or 10g Grid, because RAC is what allows the database to participate in the 10g Grid.
Each node in the grid that runs RAC needs an extra RAC license. Additionally, each node must also be licensed for the Oracle database.