There are differences between mono and fluoro that no one has mentioned.
One is water absorption. Mono absorbs water gets weaker. Fluoro does not and maintains it's strength.
Monofilament is porous and flouro is solid so that for a given volume, there is more material in fluoro line. Therefore, for a given X size, flouro has the potential for being stronger than mono. Check out the
line strength ratings of Rio mono vs fluoro tippets and you will see that the fluoro is stronger than the mono. The "rio" difference though is in wet strength. Mono will get weaker and fluoro will maintain its strength. The absorption of water by mono also weakens the mono knot including the tippet to fly knot.
In dirty water, there probably is no difference in visibility between mono and flour. But in clear water there is. I agree that fish primarily do see drag and that is the #1 reason fish will refuse a fly. But fish also do see the leader. Some fish ignore it and we will catch those. But what about the big fish that refuses? We cannot say that what the reason is. Could it be that it saw the tippet?????
I've read reports of fish spooking from tippet material, specifically the sheen of tippet and the shadow from the impression of the tippet on the meniscus. Tippet and leader resting on the water, depresses the surface slightly in an irregular fashion. There is a difference in light refraction between tippet and water and so the tippet and leader cast both a shadow on the bottom of the stream and light "sparkles" from refractions. These fly fishers "degrease" their leaders with a
fullers earth compound to remove the sheen and dull the surface. They also sink the portion of the tippet near the fly to reduce these effects.
This has convinced me that for
some fresh water fisheries, leader visibility is not only important but critical to catch the largest and most difficult trout.
For salt water fly fishing, I think it is pretty "clear" that fluorocarbon is more effective for the same reason that clear fly lines are more effective. Using both clear lines and fluorocarbon leaders in fresh water minimizes the variable of line and leader visibility.
Modern fluorocarbon tippets are limper that the older fluorocarbon tippets. They are limper than fluorocarbon fishing lines. They are also stronger for a given diameter. I use the fly fishing tippet version of both mono and fluoro because I want the strongest material, but that choice is yours.
Mono weakens with UV exposure, fluoro does not, so again for salt water fly fishers, there is an advantage for fluoro. They can use the same material year after year and buy it in bulk.