The closer the vented loop is to the exhaust/mixing elbow the less water there will be that falls down into the mixing elbow when the vent breaks a siphon. This, in turn, means there's less water that has a chance to make its way back up the exhaust
system while the engine
is off and potentially cause a lot of grief. If you have a fair sized water muffler
or similar in the exhaust
then the the volume difference is probably pretty negligible in comparison.
Bottom line, preferred is after the HX (for the reasons stated) but before the HX can work
just fine. On some engines the connection from HX to elbow is so tight/special that trying to insert a vented loop there is somewhere between an exercise in frustration and impossible.