The great debate over bWAR and fWAR. Offensively they are pretty close, but Pitching wise, I'd always use fWAR as it uses FIP instead of ERA. You can have a really lucky pitcher with crap peripherals who gets a good bWAR (Roark last year tallied a 3.0 bWAR) but has a worse fWAR (Roark had a 1.9 last year according to fWAR).
With that, Gio was roughly just about what the Nats got out of Roark last season. Hellickson was worse because he was hurt, but also his FIP is just around what Roark and Gio had. Given Slate's list, I'd say that only Miley and Buchholz are better than Roark. Everyone else on that list is roughly the same as Roark. Now take into account the salary Roark is projected, everyone on that list is more affordable, so the Nats could trade out Roark's salary for a lesser one for roughly the same production or better.