Where exactly are you based?

I presume you are saying almost all your travel is between NYC and LON? You have to consider the parner airlines strongly when you choose to move FF program as it is unlikely you will restrict all travel to one airline. I think continental's membership of skyteam gives you far more practical travel options than does virgin's partners. With CO for example, you have choice of CO, DL, NW, VS, AF, AL, KL while for VS I think its just VS, CO, DL, SAS and BMI. I think if you travel globally then skyteam may be a better option and also same if you travel within North America (because of NW). However if you mainly just travel between UK and US then it doesnt really matter.