T.H.
So my kids are exactly 2 years and one week apart (my second was born on my first's due date incidentally) and I was in the same boat as you. I really thought I needed that double stroller and was looking at different options. A friend of mine had a double front/back that she let me borrow for about 3 months and honestly, that was all I needed. I had to give it back to her and I decided to just wing it with my one stroller (compatible with the carseat) and it was perfectly fine. I'm so glad I didn't spend the money on a new one of any kind!
So, my first advice would be to see if you can find a used one on craigslist and go from there. The front/back are a lot easier to maneuver through stores but once the younger one gets big enough to look around they won't want to be back there all the time. The side/sides are just bulky unless you're going to use it for jogging. If you are going to get one I'd probably go with the sit and stand.
Good luck!