Rain delay research: 1975 Red Sox vs. 1978 Red Sox...
WAR -- 31.9 '75, 24.5 '78... seems odd because bb-ref's top 12 Sox for '78 rocks WAR, but remember, that team had zero bench contributions because Zimmer was afraid to take his foot off the gas... '78 had seven guys at 3.4 WAR or higher -- Rice, Eck, Fisk, Looie, Lynn, Stanley, Torrez... compared to only two in '75 -- Lynn and Evans.
The '78 squad had eight guys qualify for the batting title (502 plate appearances), twice as many as the '75 guys. The '75s had nine with between 100 to 339 PA; '78 had three. The pennant winners were a more complete team. I'd take '75 Cecil Cooper over '78 Boomer Scott, and '75 Denny Doyle over '78 Jerry Remy.
On the mound, '78 had more studs. Eck was better than Rick Wise, Torrez had a better year than Spaceman, Steamer outpitched Moret, and even '78 Tiant doubled the WAR of '75 Tiant... but that was regular season Tiant (his postseason will always give the younger Looie the edge).
On defense, it was most of the same guys -- except at 3B, where Rico Petrocelli in his final year in '75 was still dependable, compared to Butch Hobson, whose bone chips in his elbow led to 43 Es (not a misprint).
Bottom line: Wins -- 102 '75, 99 '78... yes, I'm counting the postseason -- the most important season -- because '75 got there. '78 didn't, after choking off a big regular season lead, then losing a showdown by one run to the defending champs. '75 didn't choke, but swept the three-time defending champs, and then fell in the final inning of the final game of the World Series by one run to one of the greatest teams of all-time.