Both of the O'Fallons are suburbs and come with everything bad and good associated with suburbia.
Our commutes are bad only by our standards, not by those of a major metropolis. It's likely our idea of rush hour would make you laugh as much as our winter snowfall totals.
O'Fallon MO, near where Mastercard is located, is definitely more suited for couples who plan on having kids, families and empty nesters. It has a population verging on 80K, which is big for us.
Young, single folk tend to live in either St. Louis City or in one of its inner-ring suburbs, close to the nightlife they like. However, doing so would string out your commute, either via I-70 or I-64.
One compromise might be to live in St. Charles, which has its own microcosm of nightlife along Main Street. Then you could visit St. Louis City on your own time, without the hassle of an extended commute.
Regardless of where you land, you'll pay a pittance for rent compared to what you'd pay in NYC or NJ. It may be cheap enough that you wouldn't need a roomie, unless you desire company.