Hi J.,
My son was 20 months when my second was born. I opted for a side by side walking stroller for the park, zoo, walks, the state fair, etc. I carried the baby in a sling or bjorn for the first 6-7 months. By then, my son was about the age yours is going to be, and he really did well walking most of the time. We do most of our walks in the neighborhood so the side by side was perfect for that.
I expected him to walk when we went shopping, or were going to or from a building. I thought it was good for him to learn that behavior early, and I had him walk a lot the last few months before baby was born so he was used to the rules.
I was going to buy a sit and stand, but when I really thought about how often I would use it (vs. a walking side by side) it made more sense for me to get the walker.
Make a list of the places you plan to use the stroller. If you go shopping at the mall often, and want a stroller for the kids, then you need something navigable in smaller spaces. Also think about how independent your daughter is, will she be willing to walk or want to walk more often than not? Then maybe you don't need the double stroller.
Something else to consider is what season it will be when your baby comes. Because you are due in the fall/winter you may end up carrying the car seat a ton! Maybe you just need one of those things your carseat clicks into so you can push it easily and hold your daughter's hand.
Good luck with your decision!
J.