I think it's worth considering both, particularly if your rig can handle it - I can't tell from what you posted.
I tried ROF on my dual core setup, and it was pretty horrible, so I'll be waiting until I have a very chunky quad core before I buy it. By then, I hope, the campaign will be up to the standard of OFF (or RB3D).
I do own OFF, and I can tell you that it is deeply addictive, particularly if you like the early (1916-17) period that ROF doesn't yet cover. OFF P4 (phase 4 - it's currently on its third iteration) is due at some point in the future, although the devs are keeping the release date close to their chests. I would recommend a scoot over to Combatace site to check out what's going on there. Also, you can do a lot worse than looking at the many Youtube vids that show footage from both ROF and OFF.
Finally, it would be most remiss of me not to mention ArgonV's excellent - and free - WWI version of Screaming Demons Over Europe and First Eagles 2, both of which I've played and enjoyed to some degree. And, as an addendum, at some point, Canvas Knights - the WWI mod for IL2 (I think? Please someone correct me if I'm wrong) - will be out, which will add yet another choice for you.
Best of luck in deciding!