Motherboard has dual LAN jacks, which should I use?

routers/switches with multiple LAN slots for RJ45 make equal use of all slots - -
you can use any, some, or all without concern that there might be differences in the usage.

The WAN slot (on routers) or sometimes the UP LINK on a switch does matter;
they both go to the TCP Source that is to be share via the LAN.
 
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