The 489 is the strongest, it has a 1.78" pinion, but its tapered and uses a crush sleeve. So, some folks like the 742 better, it has a 1.75" pinion, no taper, no crush sleeve. 741 is the weakest of the 3, it has a 1.375" pinion. That said, its plenty strong enough unless you're running a big block at the track.
The 742 is most likely to have a clutch type Sure-grip, followed by the 741. The 489 was later, and used mostly the cone type sure grip. More parts and gear ratio's are available for the 489 and 742 also. The only reason I can see to look for a 741 over a 489 would be that it would be more likely to have a clutch type sure grip, but, its also less likely that it has a sure grip at all. And the 741's are usually loaded with freeway gears, both my Challenger and my EL5 Dart had 741's with 2.76 gears.