When we had a 3yr old and a new born we tried both (Callum has dyspraxia so walking more than 100 yards without cut knees and bruises was impossible).
Side by side was great because the baby can see more, but the weight difference meant that it used to have a mind of its own and go off in all directions. By the time we'd used it for 3 months the frame was warped and we bought the tandem.
In the tandem, Charlotte couldn't see what was going on until she could sit up, but it was so much easier getting in and out of shops and steering. It's hard work getting up pavements because all the weight is at the front, but it's ok once you get the technique of doing a bunny hop on the handle. Once Callum started school, we passed it onto friends and they used it for a good couple of years.
Tracy xx