Search this and other pprune forums. A lot has been said, by me and others, about OFT and in general the difference between flying here and there. A fellow ppruner named SoCal Approach also has made a fair bit of comments on everything regarding the TSA clearance, M-1 visa and such. Make sure you've got all the paperwork done, and learn as much theory before you go there. Try to avoid hurricane season (roughly april to october), if you have to go mid-summer go to California instead (Anglo-American Aviation I think the place is called).
Short version on when you get back: Yes, there is a difference. Aerodynamics are the same in the UK and US, but legally, airspace, R/T, there are differences. Once you get back with that shiny PPL, book two hours ground school with an instructor, one-on-one, prepare (weather, notams, flight plans) and fly a mock flight (particularly the mock R/T to get a FIS/RIS/MATZ transit). Then book at least two flights with an instructor, both as a club checkout and airspace+R/T familiarization. Then build your experience slowly, solo first, then with passengers. As long as you are aware of the differences and willing to learn (which you clearly are by visiting this forum ahead of time) you should not have a problem.