I can imagine older fans favor Yaz, who could play the field well (7 Gold Gloves) and probably didn't take PEDs. Younger fans probably don't care about defense, and only know Yaz as a collection of stats and newsreel footage, whereas they can remember with vivid detail Ortiz's big hits, and the celebrations they sparked. Yaz has Gold Gloves, Ortiz has rings. Yaz had to deal with a Curse, Ortiz ended it.
Some think that Yaz is a better all around player, but they'd take Ortiz in a clutch situation. That seems to be a popular opinion.
Neither are right. Neither are wrong. Trying to argue about which is better is a fool's debate, because each's greatness is unique and different and not comparable.
Ortiz will be in the Hall of Fame because of his postseason heroics. The 450+ homeruns he hits will tag along, but they're not the reason he'll be inducted. Ortiz has the rings, he has the big hits, he spearheaded the 2004 comeback. Ortiz's homeruns don't have much to do with his future enshrinement. Yaz, on the other hand, is in Cooperstown BECAUSE of his 453 homeruns, along with his other offensive stats.
The fact is that until recently 450 homerun hitters were uncommon. These days, hulking, defensively challenged 450 homerun hitters with questionable blood chemistry are a dime a dozen. Paul Konerko has 439, Adam Dunn 454, Jose Canseco 462, Carlos Delgado 473, Gary Sheffield 509, Manny Ramirez 555. In an era where Jim Thome has 612 homeruns, cracking 450 just isn't a big deal.
Yaz and Ortiz have similar HR totals, but the reasons they are great are very different. Yaz was great for 15 years. Ortiz has been great in October. Those are the reasons they'll be hanging out in Cooperstown together.
Funnily, Yaz also has great postseason numbers (17 games, 4 HR, 11 RBI, .369 average, 1.047 OPS). Ortiz also has had a number of great regular seasons. However, these are not the reasons that the two are considered great. So arguing about which was greater is a foolish exercise, since they are great for different reasons.
It's like comparing Ty Cobb to Babe Ruth. One was great because he hit for a high average and stole bases, the other was great because he hit homeruns and won World Series. Ruth had a .342 average, 10th all-time, but that's not why he's considered great.
Compare Rickey Henderson, a leadoff hitter, to Ken Griffey Jr., a middle of the lineup power hitter. Compare the defensive genius of Ozzie Smith to the batting titles of Tony Gwynn. Compare the greatness Tom Glavine, who never led his league in ERA, to the 7-year dominance of Pedro Martinez from 1997-2003.
Now people want to compare Ortiz's clutchness in the big moment with Yastrzemski's consistent quality over a long period of time?