Hardware Tab: is what the server will see as it boots up. so if it set to 2GBs it will say 2068 or whatever in the guest OS
Resource Tab: Is where you can make a reservations and limitation on CPU and RAM. Example: You want your GuestOS1 to have a minimum of 1GB of the 2GB of RAM. It will make a reservation that the GuestOS1 will always have at least 1GB of RAM.
This is the way I understand it from reading and setting up my own ESXi servers.