Well first off let me start out by saying that I think both are good. They've both got plenty of strengths and weaknesses, a fair share overall. Second, I own both of them and I'm really not preferential between the two. I did not buy OFF, however; I had my cousin mail me his copy and I installed the files onto my computer.
For flight model and realism, I prefer RoF. You've got to manage your engine all the time or risk it dying on you, you've got to really watch your speed and rate of descent because your wings will break off very easily (not to mention realistically; have you seen any screenshots of the damage model yet?). I've never really come across any of that in OFF, perhaps because it's running off the CFS3 engine. It's fun and challenging-- fabulous, to sum it up; probably as close as many of us will ever come to actually flying one of these machines. It's not exactly user-friendly for newcomers to our genre, but for experienced flight sim gamers, it's not hard to adjust to.
AI... I'd say they're about evenly matched. Apparently, RoF's AI system is controlled directly from the parent servers at Neoqb's headquarters (an advantage, IMO, simply because it allows the AI to actually... "learn", shall we say). However, RoF does not allow you to customize the level of AI difficulty; OFF does. Additionally, gunners in two-seaters in RoF are a little too precise in their aim, IMO. When they get a bead on you, I would advise very strongly you break off. Otherwise, they'll mess your engine and YOU up pretty damn badly, lol.
Campaign. No competition: OFF. RoF's campaign is fun, but it just lacks the level of immersion that I get from OFF what with filling out claim forms for kills, seeing specific pilot details, etc. It's still not Red Baron 3D or Full Canvas Jacket (my beloveds, lol), but it's damn good. Mission structure is just as good as well.
Graphics and sound I'd also give to RoF, but that's because it's running off a newer and more modern engine compared to OFF, which uses the CFS3 engine from 2002. It's just beautiful to look at and listen to, quite frankly. I'll upload two screenshots into the attachments section, one from RoF and one from OFF to give you a comparison. I'll also give two videos from YouTube to allow you to compare the sound-- again, one from RoF and one from OFF. I once again prefer RoF, however, simply because the engines sound exactly like the ones they're modeled after (because Neoqb worked in collaboration with several museums including the Smithsonian to collect sound files of them to use, not to mention general information; check out their blog sometime).
Again, I would also have to give damage modeling to RoF. The system it's using is completely new. It's sort of like a ragdoll physics thingy... but for planes, lol. Again, haven't you checked out the screenshots of it? The unfortunate part of OFF is the limitations of the CFS3 engine have made its damage modeling very blockish and unrealistic to say the least. The thing about RoF is that when you bend a wing, you individually bend certain parts of it creating a very realistic damage model. When you fire a tracer at a strut and break it, you see it waving around in the wind in a dynamic motion depending on how your plane is traveling at (speed, direction, whether or not it's doing an evasive maneuver; you get the point).
However, RoF fails where OFF succeeds in terms of content. There are only four planes to fly (the Fokker D.VII, Albatros D.V, Nieuport 28, and Spad XIII; you can buy others, however, for like $4 from their website such as the Pfalz D.IIIa, Nieuport 17, and Se5; they work on more all the time and we'll hopefully be seeing a Gotha G.IV before long!), there are only about 15 paint schemes per plane to choose from (similar to OFF's pre-made system, where you pick a scheme that the devs made that you want to use; at least OFF has dozens upon dozens to choose from though), and like I said, the campaign isn't too interesting. Really, the main part that it shines through with is when you're actually flying and fighting.
I should be writing more, but I'll let someone else come along and discuss this for a bit. Pictures that I promised are in the attachments (RoF is the first one, OFF is the second one); here are the YouTube Videos: