When my oldest (now 5) and my second child (now 2) was a baby, I had a tandem double stroller. It was OK, but difficult to push, especially with my older child up front (baby carrier in the back.) But once my oldest wanted to walk around more, I just went back to a single stroller. The age difference between my second child and my third child is only 18 months. Once my third child (now 9 months) was able to mostly sit up on his own, I ditched the tandem stroller and got a side-by-side stroller. (... shouldn't put a car seat in the side-by-side, at least, not the one I have) Yeah, it's wider than the tandem, but I can go through all doors and can get around the stores. Aisles are usually the same width as doors, so it's tight but I can do it. Manual doors can be cumbersome, no matter what kind of double stroller you have. I can recline both seats independent of each other, and the side-by-side can turn on a dime, where the tandem cannot. Unfortunately, my side-by-side does not have a basket (I loved the basket in the tandem!) as I chose the side-by-side that is almost like an umbrella stroller so I could save space in the back of the Expedition. Yes, the tandem stroller was BIG! If you have a car, the tandem will be the only thing in your trunk. I have an Expedition and had to make sure that the 60% seat was folded down to accommodate the stroller. The tandem stroller I had was also much heavier than my side-by-side.
As a note, I did use a sling for a little bit with my third child, but I didn't feel natural using it. I just couldn't get it to wrap correctly and always held onto my child with one hand. I know other women who are fantastic with the sling. They are pros at using it, and once they have the baby wrapped in, they push an older child in a single stroller. I do envy them, but as I said, I just couldn't get the hang of the sling- my loss.
I would say get a tandem stroller in the beginning so you can put the carseat in it, then sell it on Craigslist once your baby outgrows the carseat and use that money to offset the cost of a side-by-side (if you will need it at the time. I am out about 8 hours a day, so my 2 YO does get tired of walking and wants to sit down.) (note- my tandem was $150, my side-by-side was $80)