I wasn't clear in my own mind about the distinction in meaning between these two words, partly because I have seen a heart attack referred to as both a myocardial infarction and a myocardial infarct. But it seems the latter usage is not strictly correct. An infarct is a localised area of tissue damaged by loss of blood supply. An infarction is an event causing an infarct, in other words an interruption to blood supply. The Merriam-Webster online dictionary doesn't even give a definition of infarction, just listing it under infarct as a derived word.
As people have mentioned, infarction is almost always seen in the expression myocardial infarction: a blockage of blood supply to the heart, otherwise known as a heart attack.
There's no doubt that the current treatment of infarct as more common than infarction is the wrong way around. Infarction seems to be used an order of magnitude more often. I'm convinced infarct should become rare, but this does not necessarily mean infarction should be common. It is a technical term. However it appears quite often in the news media in stories about health, or in reports of coroners' findings. I think most people would know the word, even if some are not sure about exactly what it means (as I wasn't).
This is backed up by the stats from the puzzle that inspired this request. Infarction was played more often than a dozen common words. Only one other rare word was found by more players - coir, which was switched from common to rare some time ago, a move questioned by some forumites.
I think on balance infarction should become common.
