They are very close, but Taijuan Walker has more ultimate upside, as he edges out Fernandez in both athleticism and raw 'stuff.' Walker has ideal, near-perfect physical tools; big, durable frame, tops out at 97 and holds velocity deep into games. His fastball, curve and change are all potential plus pitches. His problem is consistency. His FB gets flat at times, he sometimes short-arms his curve, and his changeup is still raw.
Fernandez, on the other hand, has a more developed arsenal; a fastball that sits 94 with explosive movement, a two-seamer, a hard 3/4 breaking ball, a true slider and a changeup that flashes plus. He has from all reports an incredible work ethic and maturity, and in terms of becoming a pitcher who knows how to actually pitch as opposed to throw, he's ahead of Walker on the development curve.
The upshot is this: If they both max out their tools, Walker has a touch more potential. But really you're talking about degrees of #1 starter.