DPS is the job of DPS.

[Neophyte]

Tank 1 can hold aggro and mitigate content perfectly well, while doing 3500ish dps.

Tank 2 also holds aggro and mitigates, while doing 2500 dps.

Which tank is preferable to have in your party?

This.

Second ones are like cars with broken radio, air conditioning, signals and only one gear. Sure, they still do their job (driving), but they're still broken.