At first I thought I wanted a Graco tandem double so that my infant carseat would snap in. But after trying both types at the store, pushing them around the aisles, folding and unfolding, etc., I decided the tandem strollers were too bulky and felt like pushing a bus. So I opted for the side-by-side, which felt like a wide load, but was better than a bus.
Turns out, once I started using it, I LOVED my side-by-side! The one I chose folded pretty compactly and was narrow enough to fit through all doorways except the smallest of dressing room doors. (Be sure to get one that is *less* than 30 inches wide.) Also, the seats reclined fully so I could put my newborn in without worry. (I would have only used the infant carseat about 7 months anyway.) It maneuvered well and lasted a long time, and I was able to resell it and recoup more than half the cost (and I had bought it second hand to begin with).
But the best thing about having a side-by-side was watching how my children interacted. Big brother would reach over to hold baby sister's hand, help her with her blankie, or retrieve her pacifier. And as they got older they would talk to each other and keep each other company. And then they got the the stage when they could both get in and out on their own...bliss!
By the way, my children are 21 months apart, and I used the stroller for nearly 3 years. I had a Maclaren. A friend of mine also chose a Maclaren for her double stroller and loved it. I have another friend who had a Combi Twin Savvy and loved hers too. I absolutely loved having a side-by-side and would do it again in an instant.