I've been to both and it really depends on where your tastes lie. Farnborough is good in its own right but in my opinion is more important for the tradefare and commercial sales that go on at the airshow. RIAT is just a massive, purely military airshow - you wont see ANY civvy stuff there. I am in the Air Force so am a bit biased but I would say go to RIAT... it is a great show with the best military hardware in the world and if I recall correctly, is the biggest airshow in Europe.