In this article I will briefly describe the difference between MASQUERADE and SNAT.
MASQUERADE. Less fast routing than SNAT with mass requests, since for each new connection an IP address on the external network interface (WAN) is determined. Great for home use routers and when changing the IP address on the WAN interface.
SNAT. If the IP address on the external network interface (WAN) is static and does not change, then it is desirable to use SNAT. The firewall rules specify one or more IP addresses, thus it is possible to distribute local IP addresses and networks to external IP addresses. SNAT is good to use on access servers.
See also my article:
Ubuntu IP Masquerading (NAT)