I don't see how you get to either conclusion.
If you have the Lakers, Celtics, and Bulls in front of the Spurs, you have to put the Sixers ahead of them too.
The All-Time Sixers starting lineup would be better than the Spurs, since, at worst, the following are true:
Chamberlain > Robinson
Barkley =< Duncan
Erving > Elliott
Iverson = Gervin
Cheeks = Moore/Silas/Parker
The Sixers bench would clinch it for me. An All-Time Sixers team would have 4 Hall-of-Famers on the pine:
Billy Cunningham (21.2 ppg, 10.4 rpg, 4.3 apg, 1.8 spg, .5 bpg)
Hal Greer (19.2 ppg, 5.0 rpg, 4.0 apg)
Moses Malone (23.8 ppg, 11.8 rpg, 1.2 apg, 1.0 bpg -- worst year in Philly)
Dolph Schayes (18.5 ppg, 12.1 rpg, 3.1 apg)
They'd also have better role players than a Spurs all-time team.
Need a shooter whose not a scorer? Doug Collins shot better than 50% from the floor for his career, while averaging 17.9 ppg.
Need a defender who can give some offense? Bobby Jones was First Team All-NBA defense from 1977 to 1984, was Second Team All-NBA defense in 1985. He also bothered to average 12.1 ppg (on better than 55% shooting for his career), 6.1 rpg, 2.7 apg, 1.5 spg, and 1.4 bpg.
As for the greatest defensive unit of the All-Time teams, off the top of my head, I'd argue that the Bulls All-Time team would be defensively better on the perimeter than the Spurs All-Time team. It's not just Jordan and Pippen, who are both Hall-of-Fame quality defenders, though. You have to add in guys like Jerry Sloan, Norm Van Lier, Bob Love. Put a shot blocker like Gilmore in the paint and that becomes a very formidable defensive unit, but also one that would be good and consistent on the offensive end. I'm not sure you could say the same of the Spurs bunch.