From all accounts, Gehrig was shy, modest, hated parties, shunned the spotlight and lived with his parents until he was 31 years old, when he married Eleanor Twitchell, his first serious romantic involvement.
A-Rod, of course, is A-Rod. If you need any more details than that, you need to get out a little bit, or at least leaf through a gossip mag in the checkout line every now and then.
And yet, Rodriguez and Gehrig share a spot atop a very exclusive list, because they apparently had at least two things in common:
Both loved hitting with the bases loaded.
And both were pretty darned good at it, although this season it was pretty easy to forget that about A-Rod.
In fact, it had become so easy to forget that there was a time when Alex Rodriguez, not to mention the rest of his Yankees teammates, were actually good in that situation that when he stepped to the plate in the eighth inning with the bases loaded Tuesday night against Atlanta, it seemed more likely he would fail than make history.
With his knack for being at the center of the most dramatic moment of any game, it was obvious that this at-bat would be the one we would talk about after the game was over, regardless of the outcome.
And when Braves reliever Jonny Venters missed badly on his first three pitches, it certainly appeared that the best anyone could hope for was a bases-loaded walk and a one-run dent in the Braves' 4-0 lead.
Then Rodriguez took a strike down the middle. Then he fouled one off down the right-field line. Next, he fouled one off his own foot, collapsing for a moment to one knee in the batter's box.
At that point, a betting man might well have wagered that the Yankees' team-wide .149 batting average with the bases loaded -- and Rodriguez's personal .100 BA -- was about to go down, as were the Yankees.
But then, Venters, who had ended Monday night's ninth inning by striking out Robinson Cano and Mark Teixeira, left a sinker up and over the middle of the plate. In the time it takes for a rocket ship to traverse 350 feet of green grass, Gehrig had company atop the list of baseball's all-time career grand slam hitters.
And, oh yeah, the Yankees had tied a game that just seconds before they appeared to be sure to lose.
Rodriguez's 10th home run of the season -- and 23rd slam of his career -- was a laser shot, a low line drive that looked for a time that it would knock the wall down rather than clear it. But when it disappeared above the rim of the left-field fence, A-Rod allowed himself a little fist pump as he rounded first base.
Down below in the clubhouse, CC Sabathia, who had worked the first seven innings and left his team in that 4-0 hole, felt the building shake -- there were a ton of Yankees fans among the 41,452 at the Ted -- and knew that someone on his team had done something good.
And in the stands, a 15-year-old kid who used his Yankees cap as an outfielder's glove to snag the drive on the fly had a thought: Boy, it would be cool to give Alex Rodriguez this ball back.
What followed was, in a way, anti-climactic. Two batters later, Nick Swisher hit a two-run home run. The Yankees went on to win 6-4, their fifth victory in a row and 15th in their last 18 games, and -- coupled with the Mets beating the Rays down in Tampa Bay -- moved into sole possession of first place in the American League East for the first time since April 21.
But those were all secondary stories.
Once again, the main was Alex Rodriguez, who, unlike Gehrig, openly courts the spotlight and, very much like Gehrig, often shines under its glare.
"Yeah, it's amazing," Rodriguez said. "I mean, it's one that I'll never forget. I've been a huge admirer of Lou Gehrig and his history and what he stands for and the way he respects the game and the way he represented the pinstripes. There's a stoic-ness about him that I really enjoyed, and a professionalism."
Stoicism is not a quality one automatically associated with A-Rod, and there have been opponents who have sometimes questioned his professionalism, but another thing he and Gehrig do have in common is a strong work ethic. One was a man who never missed a day of work for 16 years, and the other a man who is respected in his clubhouse as one of the hardest workers on the team.
Despite his outward expressions of confidence, this has been a difficult season for Rodriguez, especially since unlike the past two years, he couldn't explain away his diminished production as the result of injury.
This season, he has been healthy and in the lineup just about every day. And yet, his power numbers are down -- just nine home runs in his first 223 at-bats, which would put him on pace for about 24 home runs in a 600 at-bat season -- and his performance with the bases loaded had been abysmal, with just one single in 10 at-bats.
At least three times this season, the man who had hit the second-most career grand slams in baseball history found himself coming to the plate with the bases loaded because the opposing manager chose to walk Cano to pitch to him. Each time, the strategy worked.
Tuesday night, the walk to Teixeira that preceded A-Rod's at-bat was quite unintentional, and yet there was little reason to think the outcome would be any different.
Although Rodriguez had hit the ball hard a couple of times on Monday night, it seemed that he would need an escort to actually leave the Ted. That is why when the ball first left his bat, not even A-Rod was sure it would clear the fence.
"I hit a ball like that yesterday and it was only a double," he said. "So I knew at least it was a double. And when it carried up, I was really happy."
Once again, the spotlight had found Alex Rodriguez, and for the first time in a long time, he didn't shrink from it. He basked in it.
"He's such an impactful player that when he comes up, you think, we've got a chance to win the game," manager Joe Girardi said. "That's why he's talked about a lot, because he can always change the complexion of the game with one swing. There aren't a lot of guys you can say that about."
Recently, you hadn't been able to say that with any confidence about Alex Rodriguez, but for one night at least, he reminded why you used to stop to watch him hit, and why, nearly 75 years after Lou Gehrig's last grand slam, he finally has some company on his once-lonely pedestal.
"It's incredible, absolutely incredible," Girardi said. "It's hard to fathom what Alex has been able to do in his career. To be mentioned with Lou Gehrig is pretty special."
To match him is even more so.