I have both the 20 and the 30 and I would say the image quality are about the same. Added resolution should buy you marginal opportunities for larger prints. To me the value in getting the 30 were the features around image quality: fast startup time, manual zoom, flip out screen, wheels allowing on-the-fly exposure adjustments and improved rez on the EVF and LCD for more accurate manual focus. I also love the larger grip, it make a huge difference to me, personally.
With all quality aspects equal, these improvements made getting the shot easier and faster with nominal differents in the actual results. I would've been just as happy with the 30 if it were the same resolution as the 20 (maybe even moreso).
I have noticed that skin tones seem to be more accurate with the 30 but that could just be my impression and I do like the added crop area that 8mpx provides.