I use my own phone when in US for convenience to me and others (who I may not have contact with before I leave the country), and means I don't have to carry around 2 phones (I also use my phone as a backup camera). My limited use of my phone at botcon means my international call costs end up being very low, and probably less than the $50 disposable phone cost. But weigh up your options based on your own needs, because if you are spending a fair bit of time in America with others whom you need to contact often, or doing lots of other stuff which has you needing a phone a fair bit as well, the disposable option would end up being cheaper.