As you have identified, Series B has the thumbwheel switch to set the last octet of the IP address.
If the thumbwheel is set for a value between 1 and 254, then the 1794-AENT/B uses the Class C Private Subnet address 192.168.0.xxx.
If the thumbwheel is set for 255 (or 000, I think), the 1794-AENT/B works just like the Series A unit; it defaults to BOOTP/DHCP or uses whatever value is stored in nonvolatile memory.
The Series B hardware has some improved grounding and shielding, but I don't know of any significant performance differences.
There is no "default" IP address for the 1794-AENT; if the device were to lose its nonvolatile configuration it would revert to BOOTP/DHCP.