Yikes! I really should make and effort to visit the fforwm more often!
My belated opinion having done both
NYC Heathrow Manchester (then trains)
is DEFINITELY Dublin Cardiff!
As @joanalciati says Dublin is calm & clean–easy peasy. and the Cardiff airport is small so it’s nice and calm as well!
Quick tip–bring PAPER euros if you need to buy anything during layover (coffee etc.) They didn’t take pounds or euro coin last time I was there!