The reason you find so many conflicting answers to the question, “What is the ideal cadence?” is because there is no one ideal cadence for everyone, all the time. The best cadence for the moment depends on a number of interacting variables which change not only from person-to-person but even for the same person under different conditions. Knowing the general trade-offs between the variables, however, and keeping in mind a few rules of thumb will allow you to get closer to that perfect spin.
As a general statement, low cadences put more of the workload onto your muscular system and higher cadence shift more of the workload onto your cardiovascular system. The idea is for you to find the point where both systems are working more-or-less equally. My first cycling coach had a saying that pretty much summed this idea up; when I asked him what gear I should be in during a time trial he told me, “If your legs are burning, shift to an easier gear and spin faster. If your lungs are burning, shift to a harder gear and spin slower. If your whole body is on fire, then you’re in the right gear”. Of course “burning down the house” only applies when you are riding really hard but the idea is the same the rest of the time, too.
When looking for that “equal effort” point, however, you always have to keep two factors in mind. One is that you should err on the side of spinning too fast rather than too slowly. This is because your muscles take a much longer time to recover if you blow them up. Your cardiovascular system recovers much more quickly. The second point is that you can only spin so fast before your pedal stroke becomes jerky and inefficient. The more you practice high cadences, however, the smoother and more efficient your pedal stroke will get at any cadence. This is why it is so important to work on pedaling smoothly at high cadences. You should ideally be able to spin at least 100 rpm without bouncing. Coaches often recommend doing granny-gear sprints in the early spring until you can comfortably hold 125+ rpm.
Body type also plays a part but not as much as you might think. Yes it is true that fast twitch muscles are better suited to low cadences and slow twitch muscles are better suited to high cadences, but this is only true until the fast twitch muscles use up their internal creatinine stores (which happens quite rapidly at high efforts). After that, both muscle types respond about the same. If the effect were really that significant you would see track sprinters, who are all fast twitch, turning low cadences in really huge gears. In real life you see track sprinters routinely turning 200+ rpm in the final 200 meters of a sprint. This high cadence allows them to use smaller gears for the initial acceleration since track bikes are fixed gear. (BTW, if you’re curious, the all time spin record of 306 rpm was set by Nelson Vails at the ‘84 Olympic Games).
By the way, just to avoid a lot of confusion for those who want to work on high cadence in the gym, a cadence of 100 rpm on a road bike is NOT the same as 100 rpm on a spin bike because of the difference in wheel inertia. For most spin bikes I’ve used, 100 rpm on the road is approximately equivalent to 125-130 rpm on the spin bike.
From my experience coaching triathletes, beginning cyclists and those who come from a running background are more comfortable at lower cadences (70-80 rpm). Experienced cyclists are more comfortable at higher cadences (90-100). Most pro road cyclists are around 95 rpm. The bigger, stronger, timetrialists are often closer to 90. In my personal opinion, no one should ever go below 60 rpm unless they are standing because of the load it puts on your knees.