They will follow the one that their nodes validated 1st. Minor wording but if for whatever oddball reason 1 fork has blocks that take longer for the nodes to come out and finish the work that says yes this is a valid then the other will win. In theory a node should validate in seconds, but if something is funky in a block that for whatever reason takes longer to validate then another one can sneak in behind it.

This is a very very very edge case scenario but it's worth a mention. If you are running a proper mining pool you have nodes distributed all over the wold behind some fairly sophisticated firewalls and security devices. So, there could be some situations where 2 blocks come in within fractions of second and even though one got there 1st it was still processed 2nd and therefor lost. I have said it a few times, this is also important to note about pools that DON'T use real security appliances they can see and process blocks that other pools have not even started to process because their front end security devices are still working on it. Once again we a taking fractions of seconds or a second or 2 at most but we have seen blocks come in back to back that quickly.

-Dave