MESA 5I25, 6I25 and 7I92

Is there any difference between these boards other than the way they connect to the PC?

Is there any performance advantage of PCI, PCIE or ethernet?

Just thinking that ethernet would probably be most versatile?

They are basically all the same capability wise

The PCI/PCIE cards allow somewhat faster servo update rates (to 8 KHz or so vs about 4 Khz max for Ethernet)
This really only makes a difference if you have a torque mode servo drive or you need multi-G accelerations
as the default 1 KHz servo thread rate is fine for step/dir and velocity mode servos for most machines

The main advantage of Ethernet is wiring ease (thin and possibly long cable between CPU and control cabinet)
and isolation of the PC from all I/O