As a third option... I prefer a side basket. It's just so easy to toss stuff into it, and it's also where I store my helmet and other gear. Kind of an odd side effect is that the basket is on the drive side, so it functions as a sort of roll cage for the delicate bits of the drivetrain, protecting the latter when I park the bike in some crowded place.

It's a Wald 585, attached with hose clamps, has lasted for 20 years so far. Wald is one of those amazing historical brands that just keeps going, making good honest stuff.