While both of these wines have bubbles, one is meant to be consumed in its youth, while the other can be held for years, or even decades, if you so choose.
Pétilant naturel wines, commonly referred to as pét-nats, are sparkling wines that are made by bottling a wine before the primary fermentation process has finished. This causes the CO2, which is a byproduct of the fermentation process, to be reabsorbed into the wine as carbonation, creating a dry, fizzy wine. This process results in both a lower pressure and carbonation level than those of Champagne. It’s the ancient method for making sparkling wine.
Pét-nats are generally meant to be consumed in the year they’re released. While the wine might be able to last a few years past that, it won’t really evolve in a positive way, just stay the same. The bottle may actually evolve negatively, losing its effervescence and brightness.
Champagne and other traditional-method sparkling wines, on the other hand, are made by taking an already finished still wine, bottling it, and adding a bit more sugar to cause a secondary fermentation to occur in the bottle. After this fermentation occurs, the cellar masters then let the dead yeast cells of that secondary fermentation, known as “lees,” hang out and add even more complexity to the wine. Often these bottles can sit in cellars for several years before the wine is even disgorged — i.e., has the dead yeast removed — and bottled for sale. This method generates more carbonation and a higher pressure, giving the wine more lasting potential.
In terms of Champagne, both vintage and non-vintage Champagnes can age well past their year of release, but vintage Champagnes are often thought to be the most age-worthy due to their outstanding quality. These wines can be enjoyed several decades after release if stored well, though some non-vintage cuvées are built to age for many years, too.