Is this as a quick holiday in the middle of filming (in which case Dublin is short changing them a bit if long haul destinations have previously been on offer) or is it the place where they are actually staying at the judges house? In which case they aren't exactly going to be seeing much of where they are anyway so flying all the way to SF, to not see it, is a massive waste.
When do they film judges houses? Because if it was over the summer - San Francisco might seem more exciting, but it isn't that warm. If they rocked up dressed for L.A style weather, they'll have had a miserable time. Whereas no doubt anyone going to Dublin in the summer realises that it might be handy to pack jeans and a light jumper.
If it was this time of year - I think I'd cry if anyone made me go near the sea anywhere this side of the Atlantic. I'm sure Dublin is a wonderful place, but I imagine it to be pretty cold right now. Even the relatively cool San Fran will seem lovely and warm in comparison.
Still might not be bikini wearing, lounging by the pool weather though. (I did complain about the cold in Hawaii, though ...)
They're both city break places with changeable weather - I'd be happy with either (but I've been to SF, never to Dublin) I'd be gutted with both if one of the other groups was off to Mauritius though. If all you do at judges houses is lounge around by a pool and learn to sing, then chilly culture capitals are not the place to do that!