You should take uncoated aspirin in the event of a heart attack. However, coated aspirin would be fine so long as you chewed it up before you swallowed it. The coating prevents the aspirin from getting into your bloodstream quickly.
In this situation, one definitely needs to take a non-coated aspirin. Enteric coated aspirins are prepared to break down in the lower digestive system rather than higher up in the digestive tract. This means that they take considerably longer to act, when what one actually requires here is the fastest acting form of the medication.
First, the whole 'take an aspirin during a heart attack' thing is disputed, so talk to a doctor. If he confirms it's ok, take an uncoated aspirin because coated aspirin take longer to dissolve than uncoated aspirin, meaning it's effects won't be as quick as it's counterpart.