You can't compare based on total yards their best years. The game was different and for the first half of Franco's career there were two fewer games per season. Most of the difference between Bell and Harris is receiving yards. The passing game was a lot more prominent when Bell played, compared to Harris.
good point about the less games and different eras so let's compare apples with apples
Let's first look at francos peak period
1974 to 1979 and where Franco ranked against his contemporaries in the NFL
1974 Franco 8th scrimmage yards with 1206
1st was 1812
1975 Franco 4th with 1460
1st was 2,243
1976 Franco 11th with 1,279
1st was 1,762
1977 Franco was 9th with 1224
1st was 2,121
1978 Franco was 15th with 1,226
1st was 1,875
1979 Franco was 10th with 1,446
1st was 2,006
Now one year in 1976 Rocky Blier had 1,036 yards so some would say well how about that? Franco had 289 carries that year. He had 262 the year before and 310 the year after. They just ran the ball more because of Terry's injuries..Of note blier had a 4.2 ypc vs 4.1 for Franco. Yes rocky blier had a better ypc than Franco that year.....both with a lot of carries
So Franco in his era in that era of offensive football he ranked 8th, 4th, 11th, 9th, 15th, and 10th in the league and his totals were quite distant from the leader
Let's look at Bells 3 big years with the Steeler (he had a 4 year run but he had the knee issue 6 games into the 2015 season so I can only look at 3 full peak years
2014 Bell was 2nd with 2,215 scrimmage yards
1st was 2,261
2016 Bell was 3rd with 1,884
1st was 2,118
2017 Bell was 2nd with 1,946 yards
1st was 2,093
So Bell in those 3 years was 2nd, 3rd, and 2nd and his total yards was close to the leader
So yeah better human being..... Franco harris by a landslide. Better career Franco harris by a landslide
But in those 3 years when Bell was the man in Pittsburgh he was at the very top of the NFL at his position. Franco.......not so much.
Franco racked up a lot of yards over a longer career, won 4 SBs and was a great guy and beloved but peak 3 year Bell vs peak 6 Harris......I'd say Bell was the better back and frankly I'd don't think it's close