I would have picked Manziel without any hesitation whatsoever.
Having seen Garoppolo now, however, my decision would be different. In fairness, this is also not the first time I'd be wrong. While I still hate Manning, I would've been wrong to pick Leaf back in the day also, but were I a GM at the time, I would've gone with him over Seyton. And when the Pats picked Brady, I actually didn't care at all. I also rallied for BB to put Bledsoe back in and called him Bill Belichicken almost the entire season for failing to do so. (We then won the Superbowl and I yelled "haha I knew it" and called all my friends and in-your-faced them... Like I knew the whole time...)
It is probably good that I don't evaluate QBs professionally.