It is really a matter of synergy and personal preference--you really need to try different approaches to find what you would personally like.
In most instances, where a suspended wood floor over carpet is concerned, I would certainly try, as an option to either spikes or soft feet, an energy absorbing platform that contacts the entire surface of the bottom of the speaker (e.g., Symposium Svelte Shelf). With these devices, the vibration of the speaker cabinet is transferred to the platform, and the soft inner core of the platform converts that energy into heat and thereby dissipates the energy. The result is a tighter sound and greater clarity.
Rubber feet on carpet would tend to be less stable so that the speaker would shake excessively and that could be a source of spurious emissions from the whole speaker. Spikes would be much more stable and would transfer the energy to the flooring below. The potential problem with that is that the floor itself could act as a large sounding board.
But, again, let me emphasize that all of the potential problems with any of these approaches are just that--theoretical, potential effects, and in fact, those effects might be just what is needed to make the sound better. Perhaps the extra bass bloom from the floor acting as a sounding board might be a benefit, not an unwanted effect. The tighter sound of using an energy absorbing platform might make the sound too dry for one's taste. The only way to find out is to give each approach a trial.
If I could only try one alternative, it would be the energy absorbing platform for any speaker that cannot be spiked to a concrete slab type flooring.
