A 20 gauge of the same make/model is most definitely as good a grouse/small game gun as the 12 would be - most likely better.
Since you have decided to purchase the 20, there's a consideration you should be made aware of. Often, when using smaller gauges of shotguns, shooters think they can have the best of both worlds by getting heavier loads. Intuitively, it would make sense that a greater powder charge combined with more shot will produce more dense patterns and longer effective range. In actuality, the reverse is true.
In W.W. Greener's book "THE GUN" (highly recommended reading for all GunNutz), he explains in detail how the powder charge should be large enough that it burns all the way down the barrel, but not so large that unburnt powder remains after the shot has exited the barrel. Otherwise, the "blow-by" that rushes past the shot stream as it exits the barrel strips the outside pellets and scatters them as "fliers".
He also explains how, for every gauge, there is an optimum amount of shot. Employing shells with a less than optimum amount of shot, obviously reduces the total pellets hitting the target and thins the pattern. Using anything greater than the optimum amount has also been proven by experimentation to result in less pellets on target and larger voids in the pattern.
So-called "game" loads are usually too heavy to fall into the sweet spot. To get the best performance out of your new Citori, look for 7/8 - 1 oz loads with standard powder charges. Finding 3 or 4 versions of similar shells and patterning them will, almost invariably, identify a make/load that outperforms all the others in a particular gun.
Personally, I'm hooked on Gamebore's "Traditional Game" loads, distributed by Kent Cartridge in Canada.
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