i've got a topeak super-tourist DX (disk brakes) rack on my bike, and it's usually carrying 1-2 ortlieb back-rollers. for extra room, or just traveling light, i'm considering a trunk-bag.

what's standing out is the ortlieb trunk bag and the topeak trunk dry-bag.

the topeak is 50% bigger and will fit onto my rack without an adapter plate. the ortlieb is probably better quality, but smaller and the side pockets aren't waterproof. i'm not sure yet if the ortlieb adapter plate will fit well on the rack i've got, and i'm not sure if the topeak bag is made with PVC (which i'd like to avoid).

anything else i'm missing in the comparison? does anyone have any opinions to share about these bags, or other waterproof trunk-bags i should consider?