GeoffC, I cannot let this stand unopposed:
1. Compatibility - vary rare to find a manufacturer that doesn't support it
Wrong. Modbus RTU has almost universal support. Modbus TCP is supported by only a few.
5. Can be Cheap - If you dont need all the above then you can use cheap office grade components. Great for testing.
If you talk about a PC that talks with a PLC that has native Modbus TCP, then yes. In all other cases a system based on Ethernet will be significantly more expensive. And I cannot imagine using office grade equipment in an industrial application. Here in the EU, you are not even allowed to do it (EMC directive).
The entire subject is something else.
There is not one universally accepted bus standard that is based on ethernet.
The only widely accepted i/o bus standards are Profibus and DeviceNet.
The only widely accepted non-i/o bus standard is Modbus RTU.
To sum it up for the original question.
The advantage of Profibus is that it is the most widely accepted standard, and you can create up to medium sized systems at an affordable price.
The advantage of any of the Ethernet based networking standards is that you can create very large and performant systems.
And you can combine Profibus and Profinet to gain the advantage of both.