Ozone vs. FabFilter - I use both, but for different things.
Limiter:
I tend to use Ozone's limiter for the easy projects that don't call for a lot of master bus manipulation. Throw it on, quickly dial in a threshold while watching SPAN's K-14 meter and you're done.
Pro-L, OTOH, is capable of more clinical tweaking. That's my choice for individual tracks and sub-busses, and for more aggressive and louder styles. Its Dynamic mode is awesome for drum busses. If had to choose one over the other, it'd be Pro-L, but not by much.
Exciter:
FabFilter's answer to Ozone's exciter module is Saturn, which is more versatile if you want severe distortion but isn't nearly as good as Ozone for more subtle applications. Even in projects where I'm using all FabFilter stuff on the master, there'll still be an instance of Ozone just for the exciter.
Multi-band compressor:
I haven't used Ozone's multiband compressor for a long time, but I like it better than Pro-MB despite the latter's beautiful UI. At any rate, these days I'll use Meldaproduction's MDynamicEQ rather than a multiband anyway, so neither Ozone nor FabFilter wins this round. I haven't used the new dynamic EQ in Ozone, but it looks a lot like Pro-MB, and Melda's UI is a lot easier to use than either of them.
Equalizer:
Ozone's excellent EQ has a slight edge on the master bus, but Pro-Q (now Pro-Q2) beats it on individual tracks and submixes. Ozone has the spectrum-matching feature, but I don't use it. Mainly, I like Ozone for the Critical Band view, very handy for mastering.
However, I still have to give this round to FabFilter because Pro-Q is lightweight enough to use on anything, with as many instances as you like. Ozone's too demanding for use on tracks, which is why they sell Alloy, which is kind of an "Ozone Lite".
Imager:
FabFilter doesn't have a product to match Ozone's stereo manipulator, so this is a bonus when you get Ozone over FF.
Price:
Ozone is a better bang for the buck, being $350 for multiple effects versus $500 for FabFilter's Mastering Bundle (Pro-C + Pro-Q + Pro-L + Pro-MB).
Ergonomics:
FabFilter's UIs are among the very best in the industry. Controls are consistently implemented between products. Multiple screen sizes. GPU acceleration makes controls move smoothly during playback.
Ozone is dark and monotone. FF wins the beauty contest hands-down.
CPU Efficiency:
No contest. FabFilter beats most products on the market in this regard.
Quality:
Tie. Nobody knows audio better than iZotope. Nobody works harder to please their customers than FabFilter.
Support:
Both are good, but FabFilter gets the nod here because the developers actively communicate with their customers via their forum. Ask them a question and you'll get a quick reply from the guy who wrote the code. Ask for a feature request and they'll tell you yay or nay. iZotope, OTOH, just says "that's the way it is, live with it".
Summation:
Although Ozone is a better buy specifically for mastering, FabFilter wins this contest overall, being appropriate for every stage of production.