I use both IP and RS232 for control. Unless I am out of serial ports, or the receiver is only reachable via IP I will always default to RS232 control, it is by far the most stable method.  That said The lower cost denons and Marantz units should never be controlled via IP as their network interfaces suck. However, they do not have a serial interface either so that is a pickle, therefore avoid the lower cost models unless you are using strictly IR. The higher end receivers are much more stable with IP, but by no means perfect. If however, you have to use IP to get a control signal to a receiver then use an ESC-2 connected to the receivers serial port as this is way more stable than just connecting to the receivers network port directly.