b.) Both of the configuration when implemented achieve the same thing.
a.) In first set of configuration Auto NAT / Network Object NAT where the user configures the whole "nat" configuration under the created "object". Such type is used to do configuration for Dynamic PAT , Static NAT and Static PAT.
The second configuration deals with scenario of "Double NAT" or "Twice NAT "/ "Manual NAT". It uses configurations to list the real/mapped addresses in the NAT configurations by
creating different "object" and "object-group". This "nat" configuration is not located under any objects but rather uses them. Typically this configuration format is used to configure NAT0 or Policy type NAT configurations.
Another major difference is the way of implementation post 8.3. The NAT configurations are divided into 3 Sections which defines their priority in the "nat" configurations
Another big difference between Auto NAT and Manual NAT is Auto NAT only does translation for the source address (might seem weird depending on which side you are looking the situation from) while Manual NAT performs translation for both the source and the destination IP address.