As well as Skyscanner, also try Kayak.
I've flown Delta and Virgin on codeshare and I actually quite like Delta. Last time I flew Virgin to Orlando in economy it was awful. I'm trying them out again this summer to the West coast so I'm hopeful following Hulababy's comment.
I've also flown United and US Airways to the US and they were fine too. For the summer holidays, I'd go for the cheapest direct flight or one with a short connection ie Dublin as suggested above.