That’s actually a really good question, and it’s not all that clear.
Thomas was a really, really dominant offensive player - by way of comparison, he has been as good a hitter as Stan Musial, Tris Speaker, or Henry Aaron. In terms of offensive ability he in unquestionably one of the greatest hitters of all time, like in the top 20 or 30. He is vastly superior to Andre Dawson, creating more runs in a much shorter career, and having some seasons that are of historically great magnitude. Thomas was very obviously the best hitter in the American League through the early to mid 90s.
The drawbacks to Thomas are obvious:
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His career, so far at least, has been very short for a Hall of Famer; 1959 games. Most Hall of Famers are well above 2000. Andre Dawson played 2627 games.
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Thomas was, to put it very mildly, a bad defensive player. He always tried his best, but at his best he was still bad. He basically has zero defensive value, so he has to get in purely on hitting.
If I use Dawson as a benchmark for a guy who’s close but not quite there, the issue is whether Thomas’s big advantage in offense makes up for the defensive drawbacks. Thomas, as I mentioned, created more runs than Dawson (1700 to 1536 - I know RC isn’t a perfect stat but it’s good, and useful for this comparison) in a shorter career. In other words, Thomas would have to play about four full time seasons and go 0-for-4 years to be an equivalent hitter to Andre Dawson.
The issue, then, is how much Dawson’s glove was worth. A really, really, really good outfielder is worth maybe 15 runs a year above replacement value. Thomas was probably worse than replacement value, defensively. Given that Dawson was pretty darned good for most of his career, although he really slowed down in the mid 80s, I don’t think it’s at all unrealistic to think that the defensive difference is worth 150 runs. You also have to consider that Dawson provided much of his offense while playing center field, a fairly defense-heavy position, though it’s not like shortstop or catcher, so I’ll give him another 50 runs for that, which I think is a bit high but it adds up to 200 so that’s a nice round figure.
If we tack that on to the offensive contributions, that’s 1736 runs for Dawson, 1700 for Thomas. But Thomas still has played 600 fewer games; 36 runs ain’t a good tradeoff for 600 games. Thomas is still better.
I am inclined to say Thomas is above the line and Dawson isn’t because A) Thomas’s career, as I think I have illustrated, is generally more impressive, and B) Thomas has much higher peak value, and I think that has to count for something. Dawson played longer but he was never the ultra-dominant force Thomas was; Thomas WAS the White Sox offense. He was an astounding hitter, one of the best I’ve ever seen. You don’t even have to account much for context; Thomas was an incredibly dominant hitter before the home run explosion.
Just to throw in another fun stat, Thomas at present has accumulated 362 career Win Shares; Dawson had 340, but again in a much longer career. That strikes me as being a pretty accurate guesstimate as to their relative values.
That of course means I am advocating the one-dimensional player ahead of the multi-dimensional player, which I am sure if the first thing you’ll complain about. Meh, I’m okay with that; generally speaking multi-dimensional players ARE better, though. Most of the greatest players in baseball history could hit and play defense, like the much-mentioned Mike Schmidt, or Willie Mays, or Honus Wagner, or Babe Ruth who could pitch, etc. etc. I would not put Thomas in a class with those men; I think he’s a Hall of Famer, but if you ranked all the best players in baseball history I wouldn’t put him in the top fifty, and he might not make the top 100 (there’s over 200 players in the Hall of Fame, which I think is about the right number, although not all the current choices are the right ones.)
As a class I’d rather have guys who can hit and field. But some one-dimensional players are better than some multi-dimensional players; I am sure you will agree that Ted Williams was a better player than Jesse Barfield, or that Orlando Cepeda was a better player than Orlando Hudson has been so far. In this specific case, the one-dimensional guy, Thomas, is better, and I’d put him in the Hall of Fame.