Much has been said about the pros and cons of ROF vs FC and no doubt everyone will have their own reasons for chosing a favourite but I'll mention one thing that doesn't seem to have had the attention it deserves:
Many have said that for single player ROF has more/bigger maps, more planes and a scripted campaign system all contriubtuing to generating a much wider varity of encounters than FC, however to put things in a very blunt and crude way ROF has articifical "stupidity" piloting your opponents whilst FC has artificial "intelligence". Since 777 appointed a new programmer to work on the AI for IL-2 things have been steadily improving with every update, patch and bug-fix.
In ROF I always felt I knew what the AI would do (which was not much) so you'd just follow behind them spraying them with bullets until they fell apart and then you'd keep moving on to the next targets until you ran out of ammunition. If you only got three or four kills from a flight you felt you had done badly, if you got seven or eight kills out of a full load of ammunition you felt you'd done well. Consequently all the encounters regardless of location, time period or enemy aircraft felt repetitively similar.
In FC the computer controlled opponents will try to use a speed/altitude advantage against you and won't easily waste that energy advantage, if you are the one that has the advantage they use all manner of evasive tactics and will massively vary their speed further trying to force you to overshoot or at least sacrifice your own energy advantage to stay on their tail. Add into that the subtle improvements to the damage model in FC meaning that it seems a little less bothered by wild spray and hope shooting that simply punches many little holes in canvas, better rewarding accurate targeting off critical systems, meaning you can't assume that your enemy's wings will soon fall off if you simply sit behind them firing enough bullets in their general direction.
All told it means a ten minute encounter in FC in which I "fight for my life" and flee to freedom with out ever having got into a position to pull the trigger on my own guns seems far more rewarding (and realistic) an achievement than the "shooting fish in a barrel" situation I associate with ROF.
ROF was, and in many respects still is, a great WWI flight sim. However I don't think I could go back now, and if Flying Circus ever fulfils the potential it is showing at this early stage by growing and flourishing slowly over time in the same manner as ROF did then it will become something very special.