One of the common questions I get is what is the difference between Masquerade and SRC-NAt? Which should I use?
The quick answer is to use SRC-NAT if your gateway IP is static, and use masquerade if it can change.
The Mikrotik Wiki Entry
Firewall NAT action=masquerade is unique subversion of action=srcnat, it was designed for specific use in situations when public IP can randomly change, for example DHCP-server changes it, or PPPoE tunnel after disconnect gets different IP, in short – when public IP is dynamic.
Every time interface disconnects and/or its IP address changes, router will clear all masqueraded connection tracking entries that send packet out that interface, this way improving system recovery time after public ip address change.