This is the most recent relevant thread I can find m I'm looking to book a trip. JFK ->lhr, LIS->JFK

If I search on Google flights I see a great flight Delta on the out bound, AF on the return. The Virgin Atlantic price is 200 dollars cheaper than Delta, the air France price is 100 dollars cheaper. Assuming I am ok with differences in rdms, is there any reason not to use Virgin (as a Delta diamond traveling with a group)? And can in actually book this with Virgin? It won't let me use Lisbon as a destination on the Virgin website.