I'd get one that is inherently watertight and doesn't need a cover. The covet is extra weight and space. Or you forget to bring it. Or you have to stop mid-ride to put it on.
I don't know the model, but people I see commuting use what are roll up drybag packs. I have a drybag backpack for kayaking I will try once rain season starts. Only downside it doesn't have the fancy anti sweat mesh panel. But if it rains, it's wet anyway.