Just personal preference. I prefer SBS if I had to choose only 1. SBS tend to be easier to steer and allow both children to see. you also have less fighting over the seats. Tandems are obviously narrower and fit down shopping aisles more easily. However, they are more difficult to steer. As the children get older, there is a better possibilty for fighting over who gets to sit in the front vs the back. On tandems, you won't be able to have both seats recline fully. Also, the seats on tandem strollers tend to be smaller. I've heard fighting can occur in either situation. Tandems, you'll get the hair pulling, kicking, and hitting. SBS you can have the pinching, sideways hitting or sideways kicking.