Goldcup, there are many ways to look at your question. One way is to look at the course(s) you shoot, and how far away the targets are when you break them. Long targets require tighter, more consistent patterns and all other things being equal, a 12 gauge will deliver better patterns than a 20.
Many sporting clays courses use medium or shorter distance targets which are well within the pattern range of 20 or even 28 gauge guns. For those presentations, the sub gauge guns are not a handicap. The ammunition is lighter and easier to haul around, and sometimes so are the guns.
Lighter recoil from a 20 gauge is not a given - perceived recoil comes from the payload and its velocity vs. gun weight and fit. A 20 gauge load in a gun that weighs the same as a 12 gauge will give reduced recoil, but if the weight of the 20 gauge gun is substantially less then the recoil will be equal to or potentially even greater than that from a 12 gauge. Eg., if you shoot 1 ounce loads from a 7 lb. 12 gauge and 7/8 oz. loads from a 6 lb. 20 gauge then the 20 gauge will have greater recoil (assuming the same velocity).
Having a lighter gun and lighter ammo to haul around can have its advantages at the end of a long course. Having a heavier gun, lighter ammo and less recoil can also have benefits. A lot depends on how you perceive the recoil and how physically fit you are relative to the course and your gear.
The choice of gun also has a bearing on the suitability of the 20 gauge for sporting clays. As mentioned by others, short barreled "whippy" guns can be frustrating on the course. I also see little advantage to having a 12 gauge gun with 20 gauge barrels on it for sporting clays (Mr. McRae shoots skeet, which has different requirements). A well balanced 20 gauge with a properly scaled frame and barrels of at least 28 inches (30" is better, some like 32") will keep you in the game. Take a look at guns such as the semi auto Beretta Teknys sporting, or the Caesar Guerini Magnus Sporting Limited U/O in 20 gauge to see the kind of configuration you are after.
If your course shows 50 yard birds or you need the widest possible effective patterns, stick to the 12 gauge. If you're an average shooter and shoot average courses, then the 20 gauge is enjoyable and rewarding. Just keep the choice of gun appropriate to the game.
Sharptail