I have both games, I enjoy both for different reasons...
If you want to be completely immersed in a believable and realistic WWI campaign environment, with probably the best AI I have ever encountered in any flying game, then WoFF is the game for you.
In RoF however, the feeling of flight is certainly more believable, and it has a more realistic and detailed engine management system... but then again, it's campaign system and AI behaviour is completely sub par compared to WoFF.
With the release of WoFF ver 3, the developers have made even greater progress in the graphics department as you can see,
(I didn't think they could top version 2, but they have), plus more aircraft, more effects, etc, but it will tax your pc system more.
If you could merge the best aspects of both games, the WWI aviation enthusiast would be in flying heaven.
For me, when I want to believe I'm flying over the western front during the Great War, then without question, I go to WoFF...
(there is an ongoing ground war going on all the time as well) plus, unlike RoF, if you stray from your flight plan, in WoFF, you are just as likely to run into enemy patrols, as there is a persistent war in the air going on in other sectors...
Having said that, RoF has it's own merits, as already described