I personally like tandems much better than side by side. They are obviously significantly less wide which makes it a lot easier moving through the crowds and you also don't take up as much of the width of the walkway. It's easier to fit through doorways (can roll it straight into hotel room, elevator, food court, stores, and any other places with a door; whereas, with a side by side it really depends on the width of the stroller and the doorway, and sometimes they don't fit, or if you have kids that like to sit with arms and legs out the sides, and you risk them bumping into the doorway, pinching fingers in the door hinges or them knocking something off a shelf in the store- I have had all those happen). Tandems fit through smaller spaces and paths in crowds as well as tight aisles in shops, they are easier to hang a thin blanket over to block more of the sun in the "back" seat if one kid needs a nap or gets overstimulated easily. I also found that side by side strollers were more difficult to maneuver when only one child was in it, or if there were two children who weighed different amounts, as the weight distribution was uneven. Tandem strollers typically weigh less and fold more compactly which is definitely beneficial when carrying them up stairs, on a bus, and then holding on to them on the bus. It's hard enough to hold onto little ones on a bus if there's only standing room- holding onto a bulky side by side stroller while ensuring it doesn't knock into anybody isn't fun. Tandems are a little easier to hold onto since they fold more compactly.
Honestly though, the crowds alone are enough for me to not recommend a side by side. It's tough navigating the parks through crowds WITHOUT a stroller. Its obviously more difficult having a stroller, and a double is especially stressful. Having a side by side makes it much more difficult to weave between people and just take up too much space in already crowded parks.
