I cut the list off at 5 WAR. The Reds have essentially had three GREAT young players in their history. Two of them peaked on the 1961 World Championship team, and the other was a major cornerstone of the Big Red Machine of the '70s. Frank Robinson hit 38 homeruns as a 20-year-old rookie and won the NL MVP five years later, Johnny Bench won two MVPs before he turned 25, and Vada Pinson was an All-Star at age 20 and again at 21. Eric Davis should be included in this elite group, but he was kept out of the starting line-up until he was 24 and then couldn't stay healthy once he did become a regular. (As is, he was less than half as valuable as Pinson through their age 25 seasons.)
Here is the same list, but only including young Reds from the current millennium (2001 on):
I removed all players with less than 1,000 PA from both tables. So basically, seven Reds have had significant playing time as young (25 and under) players in the 21st century, the most valuable being Dunn and the least valuable being Felipe Lopez and the current slugging DH for the Toronto Blue Jays. Of course, Dunn and Bruce had more significant playing time than the others on this list - they were starters at age 21 while Hamilton didn't get into the starting line-up until he was 23. Votto was a 24-year-old rookie and Lopez had his one full season as a Reds starter (and All-Star) at age 25.
Here are the same seven players, ranked by batting runs:
And baserunning runs:
And fielding runs:
Unlike the plodding, near-300-pound monster he eventually became, young Adam Dunn was actually an above-average baserunner, and while he was always a bad fielder, his athleticism overcame some of his lack of instincts in the outfield.
Anyways, if you think about it, the Reds have come up with an outfielder with a very lopsided skill set in two straight decades now. Both players were outstanding at a couple things and awful at nearly everything else. Adam Dunn had exceptional patience and power (and was a decent baserunner), and was bad at everything else. Billy Hamilton has exceptional baserunning and defense (and makes decent contact), and is bad at everything else.
Both players when they were young were plagued with questions of whether they could ever polish up their weaknesses and become well-rounded enough to be good major league players, all-the-while being so good at the few things they did well that they were above-average starters in spite of their flaws. In Adam Dunn's case, the questions continued following him as he aged until his skills deteriorated to the point where you could no longer justify his spot in the starting line-up.
But we're getting ahead of ourselves. Here are their career stats through age 25:
And their binary component rates:
Both have identical .248 batting averages, and average $H rates (BAbip) of 29%, but the similarities end there. Hamilton has been almost as bad of a hitter (OPS+ of 73) as Dunn was good (OPS+ 133).
But, taking everything else into account...who was better overall?
But Dunn had way more playing time than Hamilton, so to level the playing field, let's look at each player's WAR components again, but scaled back to one season's worth of PA:
It's worth pointing out that both players are/were above-average major leaguers, even with their obvious shortcomings (Dunn's defense, Billy's bat). The "WAA" column means that, taking everything into account, Dunn created better than one win more for the Reds per season than the average major league outfielder would have. Also, both are well above the 2-WAR-per-year threshold considered a minimum performance level for a starting position player.
But, to answer the question: they're close but young Adam Dunn was a little better.
Also, Dunn was more durable - he came to the plate at least 670 times in four of his first five full seasons. Hamilton, meanwhile, will probably fall short of 500 PA for the second time in three years as a starter.
However, while Dunn was a good young ballplayer, he had what's referred to as "old player's skills" - patience and power. Players with his skill set tend not to age as well because they already lack what most players lose over the course of their careers - hitting for average and speed and defense - and Dunn proved no exception to this rule. After accumulating 13.6 WAR in 2,783 PA (2.9 WAR per 600 PA) through age 25, Dunn produced just another 3.3 WAR over the remaining 5,545 PA of his career - 0.4 WAR per 600 PA. He was below replacement-level in his final four years, when he was the DH of the Chicago White Sox.
So if Billy Hamilton can find a way to stay healthy, not lose too much of his outstanding speed and defense, and continue to make improvements as a hitter, he could still end up being a more valuable major league player than Adam Dunn was.