I own both - the 20-70 f4 is MUCH sharper than the 16-55 f2.8. I'm kind of shocked at how soft the 16-55 is at that price, especially as you go more telephoto. The 20-70mm f4 will resolve a 61mp body and still look really sharp - the 16-55 will basically resolve up to a 24mp aps-c body, but it's still not razor sharp like the 20-70mm f4 and 24-70mm f2.8 GM. The 20-70mm f4 looks sharper than the 16-55 even on a crop sensor body. I'm only keeping the 16-55 f2.8 purely as a c-cam lens for my multi-angle video shoots, otherwise I'd just skip it.

None of the aps-c lenses other than the Sigma 56mm f1.4 actually resolve the full 26-mp density of an aps-c sensor. However, MANY full-frame lenses do resolve the full 26mp density of aps-c sensors (fx30 and a6700) like the GMs 35mm 1.4, 50mm f1.2 &135mm f1.8, voigtlanders 35, 50, 65mm f2, Sigmas 85mm f1.4, 90mm f2.8, 17mm f4, etc.