Hi Jim, I changed from a D90 to a D700 nearly 4 years ago. I havent regreted going FF. Aside to all of the advice on D600 vs D800, i Would suggest that you are unsure as to which, rent a D800 and try it. That is in my opinion the best way to find out if it is for you. If you like using the Auto modes of the D90, you may not like the D800. Then the D800 A & S modes are semi automatic with P being fully automatic and M... well manual is manual.