Ventures are slow as molasses, but they can take a pounding, are carrier-capable, and can pack some firepower. Even those default-loadout Ventures can put a hurting on most Destroyers with that Mining Blaster...
iirc, all the freighters have omni shields
Buffalos lack shields, period, and are still technically a "Freighter". However, they're about as threatening as a beached whale and go down to a single Reaper torpedo...
As a Strike Ship pilot, I'll say that amongst Freighters, the Tarsus' tend to last the longest because their shields can take a pounding and they have a sturdy Hull. Buffalos are basically free kills, and the Gemini has really thin armor and only an average hull under it's shields.