One thing to consider is that you have access to the lounge in DUB with a J ticket. You'll probably leave yourself too much time to clear security, customs, passports in DUB where you do your US immigration, and it's pretty miserable in the sterile area without lounge access. Of course you can always buy it. You also get the expedited line through security at both DUB and within DUB to cover the US immigration, so flying J on the way back has its benefits all the way through.