Heathrow or Gatwick?

Both are fine airports. Where are you staying prior to going to the airport?

If all other factors are equal, I will usually pick Heathrow because I think ( no real supporting proof) that in case of irregular operations I will have more flight options to choose from there.

If you are staying in London, don't think too much about it. Pick whichever option has the best flights.

If you're staying outside London, one of the airports might be easier to get to.

Where are you staying in London?

To where are you flying?

What airline?

We are most likely flying aer Lingus to Dublin, then Seattle. We have vouchers from 2020 on Aer Lingus.

All other things being equal you are probably better flying out of Heathrow- simply because Aer Lingus flights are far more frequent out of LHR than LGW- so if there is an issue with your LON-DUB flight, they are far more likely to have room on a next flight from LHR than LGW- to avoid a long wait for your next TA flight.

Heathrow would be my choice.

In case of problems with your flight, Aer Lingus would first look to put you on one of their flights but they might also look at British Airways as they have the same parent company.

Give yourself extra time in Dublin. You will be going through US immigration and customs there rather than when you arrive in the US.