Train connections are easy either way (from OPO or LIS). Other people have mentioned the OPO option. Similarly for LIS you would need to take the metro from the airport to Lisboa-Oriente for the trains to Coimbra.

But, even though the Lisbon - Coimbra trains might take a bit longer than Porto - Coimbra due to the longer distance, one advantage for Lisbon is that the airport is basically in the same part of the city as Oriente station - they are maybe 2km apart - so it is a very quick & easy transfer on the metro, whereas in Porto from the airport to the train station you want (Campanhã) you are basically taking the metro across the entire city (although the city is not that huge). I have not timed it out exactly but it might be a wash time-wise.

Also I think there are a few more intercity train options per day from Lisbon compared to Porto. As Palal said you don't want to bother with the slow commuter-type trains from Porto.

So I'd go with whichever option (LIS or OPO) has better flight options from HAM schedule- and price-wise, and take the metro / train from there. I have found CP to be pretty reliable. Well, when they are not on strike of course!