First, the definition of "stronger" is clearly needed here. No one will honestly claim that Siths are better than Jedi to build durable civilisation. I assume we're speaking about warfare.
In this subject, the average Sith is in fact stronger than average Jedi, taken individually. Two simple explanations for this: first, the Sith are specifically trained to be warriors and powerful, where Jedi are nod destined to be warrior per say (no saying that they do not train for combat practice, because they do, but it's not the only focus of their learning). Second, the SIth code implicitely include a rude selection: be stronger, or die. The Sith who survive is one who killed his opponent from his own faction. You normally don't encounter weak Sith, because stronger Sith killed them before you had the chance. Jedi don't kill each other for power, therefore there is more "weak to middle-powered" Jedi. Don't mean there is no powerful Jedi. But it means that if you cross a Jedi, you have more probability he's a weak one than a Sith, statistially speaking.
But that's the point and the main difference between the two faction, and the Rule of Two is the embodiment of it. Jedi are helping each other, and believe you should help the weks to become stronger, even if it's not "elite-class strong". The society in his globality is stronger, even if the strengh is no individual-based.
On the other end, the SIth believe a society is strong if it has only the strongest individuals and no weak. Hence, instead of helping the weak became stronger, they chose to eliminate the weak and only keep the strongest. This way, individuals are indeed stronger than the Republic... but there are less individual.
Is it better to have more weaker people working together to be better, or less people who puts a rude and cruel selection to only keep the best of the best ?
The second option sounds more appeling to our ego, but history suggests that the first otpion, the Jedi one, is clearly more durable ^^