Coolant leak: core plug or head gasket?

So I got a waterfall down the front of the engine - seemed to start up behind the water pump housing. The heat gauge never got over normal, but I was down about 8 litres of coolant before I noticed! So maybe it got hot enough to cook the HG or even the head?

Anyway, a fast flip through the archives here pointed to the p-gasket (poster said check the core plug at the same time, but I foolishly ignored that) at least as the initial problem, so I replaced it.

Now the waterfall is still there, though it seems a little smaller. It starts as soon as the engine is turned on (from cold). There's no steam out the exhaust. No mayo under the oil filler lid. No oil in the expansion tank. Coolant's a little fizzy once it starts to warm, but I don't have experience to know if it's beyond normal. The thermostat housing is in the way of a really good look along the seam between block and head, but no visible leaking along the front there. The engine starts in a jiffy and runs smooth and nice.

Seems a no-brainer to me to go back the way I just came and replace that core plug I ignored the first time around, but before I make another no-brainer move a quick reality check with the experienced crowd: what's more likely here? Core plug or head gasket (or head)?

I would start by checking the core plug. It's the easier of two jobs.

Duly checked, and the rusty trail begins right in the middle of the thing. I guess I should change them all now before I get new waterfalls.