Hard to call. They're Arizona's #12 and #14 prospects. They're 3 years away at best.. by that time, there may be no fans left in Tampa, and possibly no team either if no new stadium movement occurs soon.
Here's MLB's description on Velazquez:
Scouting grades: Hit: 50 | Power: 30 | Run: 60 | Arm: 55 | Field: 45 | Overall: 45
Velazquez went under the radar coming out of Fordham Prep in New York City in 2012, and he didn't raise his profile much in his first full professional season. That changed this year when Velazquez reached base in 74 consecutive games, setting a new Minor League record.
One of the keys to the record was Velazquez's improved approach at the plate. Armed with a better awareness of the strike zone, he has become a more disciplined hitter, though there is still some swing-and-miss in his game. Velazquez generates good bat speed, but his small size limits his power output. His improved on-base skills and his plus speed make him an ideal fit at the top of the order.
Defensively, Velazquez is still learning the infield after playing mostly center field in high school. He has good range and a strong arm, and he could develop into a solid defender in time.
And for Williams:
Scouting grades: Hit: 40 | Power: 60 | Run: 40 | Arm: 50 | Field: 45 | Overall: 45
Williams was one of the youngest players in the 2013 Draft class, and he had some of the best power. His age hasn't been apparent at the plate, as he has more than held his own everywhere the D-backs have sent him.
Williams packs a lot of power in his big, physical frame. His approach at the plate is still a work in progress, however, limiting how much of that power he can use in games. At his best, Williams uses the whole field to hit, and with more experience, he should be able to drive the ball out of any part of the park.
Williams was a shortstop in high school, but he was quickly moved to left field by Arizona. He has the tools to become an adequate outfielder once he gets used to the new position.
Basically, both are lottery tickets.