To pitch in my own 2 cents here - I bought WOFF (ultimate ed.) sometime in early 2017 - was fairly late to get on the bandwagon since I was usually, until then, splitting my time between RB3D and FE2 - and, now, looking back, I can say with certainty that WOFF is the best investment anyone can make in terms of WW1 flight sims.
I have all of the big three WW1 sims - FE2/WOFF/ROF - and I would gladly give ROF back and re-invest the returned money in WOFF. (FE2 still stays on the hard drive in my case, together with WOFF, since I like to mod. things.
.)
In short, if you only want to have one WW1 flight sim (and that is still being developed) - the best choice is WOFF (ult. ed.) - and then to invest in the expansion packs (such as platinum ed.) when possible/convenient for you.
Happy WOFFing,
Von S
P.S. FE2 is worth a look too if you have money to burn since all of its mods. are free and it gives a lovely sensation of flight - but the terrain (and sky) engine in FE2 (and SF2) by today's standards looks somewhat dated. Also, it is no longer being developed since the TW developer has moved on to smartphone "apps." The other "peculiarity" about the TW sims is that trees are "alpha objects" and can't be crashed into. Personally, I find clouds and terrains in WOFF to be the best of all WW1 flight sims (even more to my taste than ROF landscapes/clouds which I never really warmed up to).