Which is better will depend largely on what you are trying to achieve. Resonance does really well with headphone scenarios because it can spatialize sounds above, below and behind the listener with better accuracy than the FMOD spatializer can. There are limitations with it though, it costs more CPU to process and it sends the output directly to the renderer DSP limiting submixing opportunities. The FMOD spatializer is best for multi-channel speaker scenarios or limited CPU.
What kinds of bugs are you seeing with the Resonance spatializer?