Interesting quandary - by nature a 5W-20 generally has to consist of better base oils than a 5W-30 has to..
Does it ?
how have you determined this ?
Am I wrong?
5W-20 contains less VII and has a thicker -- yes, thicker -- base oil than 5W-30 -- because it has less VII than 5W-30, which means it can achieve the same MRV with a thicker base oil, as the VII content increases the MRV. So, if thicker is better, 5W-20 has a better base oil.
In terms of actual base-oil quality (like oxidation resistance, viscosity index, or API group), you need to consider the Noack. Since 5W-20 uses a thicker base oil, it can achieve the Noack requirement of a given certification (such as 13% for dexos1 Gen 2) using a lower-quality base oil than 5W-30, as the Noack is inversely proportional to the base-oil quality. So, in terms of actual base-oil quality, you can get away with a slightly lower-quality base oil for a 5W-20 than for a 5W-30. However, in practice, blenders will use the same base-stock slate, meaning they will use different ratios of the 4 cSt, 6 cSt, 8 cSt, etc. base stocks of a given type and quality, and the quality will be identical more or less.
All this said I prefer the 5W-20, as it will usually have a lower Noack (for the reasons explained), less VII, and a thicker base oil, not to mention better fuel economy, as long as the HTHS viscosity (2.6 - 2.7 cP) is sufficient for your application and/or driving conditions.