Both drugs work by reducing the 'stickiness' of our blood cells making it harder for clots to form, but the both work in different ways.
The idea behind using these drugs is that it increases the liklihood that a blood clot wont form in the placenta due to poorly develop arteries. Most women with previous history of PE also tend to have stickier blood than those that dont get PE. So if you have sticky blood & poorly developed arteries in the placenta the more likely that the placenta will end up with a blood clot that clould slow babies growth, impair the oxygen & nutrients from getting to baby & harder for the 'toxins' that naturally build to get back to mums circulation to be dealt with. The best analogy I can give is that with PE its a bit like trying to squirt treacle through a needle - the heparin & aspirin thin the treacle to become more watery & will be more easily squirted!
It is generally advised if a previous experience of PE has occured before 35 weeks in order to maximise the opportunity for a longer gestation. I hope this helps to answer your question.
Every Woman is entitled to understand what happened in her pregnancy when pre eclampsia strikes. I hope to be able to support that process.