Waste of tempo to initiate pawn trade? Just develop?

The classical understanding is that as a rule of thumb, rather than capturing, you increase the tension wherever an exchange is threatened and force your opponent to catpure. This is because of square control.

So Nc3, classically, is better. Why? Look at the position after move 4 for exchanging and not exchanging. If you do the classical thing and increase the tension, then your opponent exchanges, you can control of the e4 square and have a knight placed on it to boot.

But if you excahnge first, then you do not have control of the e4 square, and your knight is not as well developed.

Here is a simple illustration to illustrate why increasing tension is better than taking almost 90% of the time.

This is because, for an equal number of attacks and defenders, your opponents piece will be left on the square in question with no one challenging it.

This is why often, exchanging increases your opponents activity, and adding to the tension, while a subtle and very difficulty art, is what separates good chess players from bad chess players.

So, if attacks vs defenders is equal, if you go first, opponent will control that square in the end.  (I take, he takes, I take, he takes)