Unless you fear Mariota turning into Blaine Gabbert under center, there's no reason he shouldn't go #1. I think it's kind of silly to be terrified of a spread offense QB in the NFL today. Gabbert didn't fail because of a lack of experience in my view, he failed because he is a wimpy QB who folds like a lawn chair under any pressure. Mariota does have some issues when put under pressure, but though his play declines, his mental toughness remains. He doesn't get down on himself.
Put Mariota in Seattle and I think he'd be a pretty small dropoff from Wilson. If any. This is a rare QB prospect, and any team that's worth a damn would mold their team situation to make a guy like Mariota successful. Just like Seattle did for Wilson.
Winston could be pretty good too if he went to an offense first team that could coach him out of the WTF interceptions (or, could find a way to win despite them like Indy). I wouldn't be surprised if Kelly coveted Winston as much as Mariota, since his system is so good at reducing INT numbers.
I don't think the Cardinals have the draft ammunition for it, but Winston would be a perfect fit for Arians.
In terms of risk, I think Winston's maturity is a bigger risk than Mariota's faint Gabbert similarities. And even then, immature QBs have starred in the NFL before.
I don't think you can fault a team for drafting either one of these guys at the top. I mean, compare them to almost any QB class recently other than 2012. Sanchez, Stafford, Bradford, Locker, Dalton, Ponder, Kaep, Newton, Manuel, G. Smith, Barkley, Bortles, Manziel, Bridgewater. Put them all in the same draft and Mariota or Winston would be near the top of that heap.