The parallel cable and connectors are a bit clumsy, and the cable length is limited. Ethernet should give you galvanic isolation between control computer and machine cabinet which I consider a big plus. Also you have a wider choice of computers using Ethernet since 5i25/6i25 requires a PCI or a PCIe slot, respectively. So you can't use a small ("mini/micro") desktop computer with them.
On the other hand it seems Ethernet is a little bit more finicky to get working well - parallel seems to "just work" while there are some people that seem to have to tweak Ethernet driver and/or PLL settings to get things working (and obviously you need to configure the IP settings on the host). If you want to run your servo loop at faster than normal speed then parallel is probably the way to go - I believe Ethernet is closer to its limits at 1 kHz servo loop? Also Ethernet takes up the Ethernet port of the computer so you need either a motherboard with multiple Ethernet ports, WiFi, or to use an USB-to-Ethernet dongle for the network connection to the outside world (the Mesa card should run on its own net unconnected to anything else).
Overall, unless I had some special requirements, and everything else being equal, I would choose Ethernet over parallel. If you know from the start that you need >6 outputs (note 6 plus drive enables, not
however perhaps the 7i77 is better. Then again, as tommylight says, there are expansion cards. In the end, both options should work.
