In theory if you are a winning player, you can win more money with parlays. But if you're a losing player, you will lose money a lot faster.
In practice, with a fixed bankroll, using proper money management, you can bet more per game on straight bets than on parlays, cancelling out the increased return on investment that parlays give a winning player. So you will still make more money with straight bets.
That is until your bankroll is sufficiently large to start running into maximum wagering limits. Then you will want to maximize your ROI since you'll be limited in how much you can invest, so parlays would be the way to go.