Although Cousins has recorded three consecutive 4,000-yard seasons and Smith's career high of 4,042 was set last year, Gruden told reporters that he feels the Redskins ultimately upgraded at the position.
"Yeah, without a doubt," Gruden said when asked if Washington is better off under Smith. "I don't want to compare two players, but we're always trying to be better at every position. We got better. Alex's experience is well-noted, and his record the last five years is what it is. You could argue that all day, but we feel very good."
The Redskins shuffled a third-round draft pick and promising cornerback Kendall Fuller to the Chiefs for Smith, who posted a 50-26 mark as a starter in Kansas City.
"It's not one thing, it's everything. It's the entire body of work," Gruden said of Smith. "He's very good at the intermediate ball. He's good with the quick game. He can run zone reads, the [run-pass options]. Very exciting. The ability to ad-lib, make plays that aren't there and keep plays alive. Coaching him for the first time will be exciting because I don't think there's a limit on what he can do. He has all the things you want a quarterback to be able to do."
Last season, Smith set career highs in touchdowns (26) and passer rating (104.7). The 33-year-old has made two straight Pro Bowl appearances.
Smith spent his first eight seasons with the San Francisco 49ers, who drafted him first overall out of Utah in 2005.
Since entering the league, Smith has passed for 31,888 yards and 183 touchdowns while posting a completion percentage of 62.4 in 156 regular-season games. He also has thrown 14 touchdown passes in seven career postseason games for the 49ers and Chiefs.
Cousins finished last season with 4,093 yards, with 27 touchdowns and 13 interceptions.
A Pro Bowl selection in 2016, Cousins led the NFL in 2017 with four game-winning drives, giving him 12 for his career.
In 62 games with Washington, including 57 starts, Cousins threw for 16,206 yards and 99 touchdowns with a 65.5 completion percentage and a 93.7 passer rating.
Cousins joins a Vikings' team that finished 13-3 during the regular season and fell one game short of a Super Bowl appearance.