Whether you choose to access resources through the NES or through GridPP will depends on your personal requirements. It is common for a new user to first join a Virtual Organisation, which will likely be local to them, or will represent their field of work. While there is some cross-over, the NES and GridPP offer support for different Virtual Organisations:
The NES can also offer resources to individual users who do not wish to join a VO.
You can also see what applications are already available thorugh the 2 organisations: