I'm not a fan of nylon straps since they stretch significantly when wet. The last thing I want to do in rain is stop to readjust the straps on the rack. That said, I'm also not a fan of traditional hook style bungees. I've worn some holes in stuff sacks because of the hook. There is also the problem of the hook slipping off and flying up in and unpredictable manner. Velcro looks interesting but Velcro can lose its "velcroiness" if it gets dirty.

My choice is ball bungees (aka "tarp" bungees)



Because they don't have a hook, you lash them around the rack and stretch them towards each other, hooking the balls together. When released then fly back towards the corners of the rack but don't fall off. I don't find that the load on my rack changes significantly from day to day so there's really no need to carry different sizes