In terms of performance:
The network architecture is
- RTU/ASCII over RS-232: one master, one slave; point-to-point
- RTU/ASCII over RS-485: one master, multiple slaves, sequential polling by the master; daisy chain
- TCP over ethernet: Standard ethernet hub and spoke or star architecture.
Modbus ASCII is capable of only half the throughput of Modbus RTU because it sends twice as much data as the RTU format, but ASCII is less susceptible to timing issues that are inherent in RTU. Telephone modem communications used the ASCII extensively. The drop off in phone modem comm has seen a decrease in the use of Modbus ASCII.
Neither RTU nor ASCII on a multidrop RS-485 network can have more than one master, unless a specialized black box 'bus moderator' is employed. The work-around is typically multiple serial ports.
Modbus TCP can theoretically handle more than one master/client, however multiple masters bombarding a low-end slave running an 8 bit micro with requests/queries is not likely to be successful.
Modbus Plus is licensed and relatively expensive. As far as I can tell, it's primarily used for CPU to CPU comm, not field device to CPU comm. In fact, I'm not sure I've ever seen a field device that runs Modbus Plus.