|As great as Ortiz has been, he can't hold a candle to Yaz|
To me, this isn't even an argument. Yaz by a lot. Case closed.
Now I'll readily concede that Ortiz, at least since he's come over to the Red Sox, has been a better hitter than Yastrzemski. Interestingly enough their career batting averages are identical at .285, and their career OBPs are within a point of each other as well. Both were fantastic in the clutch, too. Still, there's no question that Ortiz, with his 84 point edge in slugging percentage/ISo and superior home run total in about 5,300 fewer plate appearances, has been the superior slugger. Yaz managed only eight seasons with at least 20 homers in his 23-year career, while Ortiz already has 13 such seasons (consecutively, I might add) in his 18 years.
That said, it must also be remembered that the long ball is much more commonplace nowadays than it was when Yaz played. For most of Ortiz's career, baseball teams have typically averaged around one home run per game, usually a little more. Back in Yastrzemski's day, that number tended to be around 0.7 or 0.8. So for Yaz to have as many 40 homer seasons--three--as Ortiz really is quite impressive. For all of Ortiz's dingers, he still has only one home run title--same as Yastrzemski (who, fun fact, shared his with Harmon Killebrew in 1967).
Looking at batting runs, Ortiz also comes out on top on a per-game basis. With 372 batting runs above average in just over 2,000 games, Ortiz has averaged a batting run every 5.5 games played. With 450 in 3,308 games, Yaz tallied one every 7.35 games. A pretty sizable difference, but also note that Yastrzemski produced next to nothing in this category over his final nine seasons, managing only 50 in his last 1,200 or so games. Before 1975 he had 400 batting runs in in just over 2,100 games, meaning he was right there with Ortiz.
So yes, Ortiz been a better hitter and run producer than Yaz was, but not by as much as his advantage in power numbers would suggest. It's really close, actually, after considering the context of the eras in which they played. Yastrzemski's greatest seasons came during the offensively-suppressed 1960s and '70s, while many of Ortiz's best years came when offense was booming in the mid and late 2000s, before the recent downturn for hitters. Thus, Yastrzemski's career adjusted OPS of 130 (134 through age 38) isn't that far off from Ortiz's 139 mark (and, don't forget, was maintained for thousands of more at-bats). Similarly, his .375 wOBA is not dwarfed by Ortiz's .390, but is rather quite comparable.
Also don't forget that Ortiz had Manny Ramirez hitting behind or in front of him during his best seasons. Yaz was surrounded by his fair share of talent, especially during the second half of his career, but never a hitter of Manny's quality (Jim Rice was great, but not quite on Manny's level). Yastrzemski's supporting cast was especially weak during his peak seasons, when Boston's biggest threat besides Yaz was Reggie Smith. Another good player, but no Manny.
But if these past two MVP debates between Mike Trout and Miguel Cabrera have taught us anything, it's that there's more to baseball than hitting. With 168 stolen bases, Yastrzemski holds a clear edge on the bases compared to Ortiz, who's managed a meager 15. Yaz was a good baserunner in his younger days and basically broke even for his career in terms of added value there, whereas Ortiz has always been a liability because of his size and lack of foot speed. That closes the gap some.
Then there's defense, which puts Yaz way over the top. A seven-time Gold Glover, Yaz learned to play the Green Monster to perfection and earned a reputation as one of the best defensive outfielders of all-time. In his heyday Yaz was like a young Barry Bonds, a complete package capable of winning ballgames with his bat, legs, and glove. Ortiz, a full-time DH, has only ever been able to win games with the lumber. He almost never plays the field and adds no value via defensive contributions. He can barely play first base competently, let alone one of the trickiest outfields the majors.
So whatever edge one grants to Ortiz for his hitting, a bigger edge must be given to Yastrzemski to account for his better baserunning and superlative defense. According to Baseball-Reference, Yaz had one 12-win season (his MVP/Triple Crown year in 1967, of course), one 10-win season (1968--The Year of the Pitcher) and a nine-win season (in 1970). Ortiz has had one season--2007--where he was worth more than six, and zero where he was worth over seven. Yastrzemski compiled nearly 100 WAR (B-R and FanGraphs) in his Hall of Fame career; Ortiz will be lucky if he gets to 50. Even if you gave Ortiz as many plate appearances as Yastrzemski, there would still be no question as to which one was more valuable.
For one at-bat, I'd take Ortiz. But for an entire game, season, career, what have you, it's gotta be Yastrzemski.