Haven't seen any comparisons between the two bar sizes so here goes. I had the Progress 20.6 mm bar in my white car. Put the 22.2mm bar in the blue car yesterday. Where the smaller bar was still a bar improvement over stock, it still provided the rear a little movement. Not really so with the big bar, the rear just feels rock solid. Turn in is a little sharper and the turning radius is even shorter with the big bar. Car tracks where you point it even better and has reduced the understeer even more, making the car very neutral handling. Believe it or not the big bar is actually comfortable. Because the smaller bar would allow some movement over things like speed bumps, and still being stiffer than stock, it had a tendency to snap the car back to center more. In this case with the bigger bar, the rear end just stays planted, so no snapping back to center.
Both bars are def way better than stock so what one should I choose? As mentioned the small bar, (many companies make them including the CTR bar), is still a vast improvement over stock because of how they tuned the rear bars in these cars. If you plan on going with springs and others suspension mods, you may want to stick with the smaller bar. I have read of at least one person getting more oversteer than they liked using the big bar and coilovers, and used the strut bar to neutralize some of it. I personally hate understeer(most feel more comfortable with it) so I'd that's what you are looking for you may want look into it.
If you plan on doing just the rear bar, or only upgrade the bars (ie strut, rsb, fsb, ect) I would stick with the bigger bar. Either one is awesome