Over Flanders Fields
Over Flanders Fields (OFF) is certainly the better value, with nearly all the WWI aircraft you could ever want right out the box. Graphically, as one would expect for a newer sim, Rise of Flight is superior, as well as having a proper flight movie recorder built in (OFF doesn't, due to the limitation of the base CFS3 engine). The missions and campaigns that one can fly in OFF though are second to none. The graphics (great sound too!) are actually pretty good considering the age of the engine. In my opinion, I think the 'WWI atmosphere' in OFF is better, the aircraft really are a joy to fly and there is a certain tension that disaster could strike at any moment and ruin your pilot career log you've been nurturing for weeks (if you're lucky that is and you aren't a member of the "Twenty Minuters Club"!)*
It goes without saying that OFF isn't a flight sim for beginners. WWI aircraft can be difficult to fly at first, but then that's the joy of it! Experience and flight-time pays off, so ultimately it's a much more rewarding and satisfying experience to land your box-kite crate safely at your airfield after shooting-down your enemy (sometimes bullet-ridden, damaged and leaking oil) with only your raw pilotage skills and mixture/radiator management keeping you alive, than to say, finish a mission in an arcade flight sim such as Tom Clancy's H.A.W.X.
As far as CFS3 goes on its own, I can't really comment as I never really spent any time flying it. There were always better sims available, such as 'IL-2 1946', 'Battle of Britain II: Wings of Victory', 'DCS: Black Shark' and 'Lock On Flaming Cliffs 2', so I don't recommend buying this game on its own unless you plan on using it with a particular mod such as OFF. I mention this because there are a few free mods available, the one that springs to mind is a nice 1950's Korean War mod.
The bottom line; if you're interested in historical WWI flight sims at all, then CFS3 + OFF is a 'must buy'!
* see BBCTV's 'Blackadder Goes Forth' episode: 'Private Plane' for explanation.