What i have found from personal experience:

* Set up: Laptop + some little stuff in a biking backpack - meaning the weight is where it should be (low), comfortable vs Rack + lightweight pannier with laptop and shirts in there.

Case 1: 80 kms ride on the relatively smooth asphalt roads - rack is better, after some 50 kms of riding with backpack I feel that i need to stretch my back quite often, which slows me down. Not a deal breaker, especially if it's only 10-20 km, but a rack + slim pannier work better.

Case 2: City streets only - I find that backpack is a better option, "naked" bicycle feels more agile, especially if you cut the corners and have to jump a little bit, less time wasted getting the pannier on and off, all the precious stuff is with me 100% of the time, nothing rattles, etc.