Hi there,
My husband and I went through the same exact thought process. Our daughter was 3 1/2 when our son was born, and still liked to ride in a stroller. We didn't hear very good things about the sit and stand, and didn't like the traditional double strollers because our daughter would grow out of that seat pretty quickly. We found a stroller that we absolutely love and fits both kids. It's called the Phil and Ted's e3 stroller. It's pretty pricey, but we bought a new one on e-bay for $150 less than in the store. It's expensive because it can be a single or double stroller, the front wheel locks so that you can use it as a jogger, the older child can sit in front and the back of seat folds down into a bassinet for the baby to sleep in. When used as a double stroller, it's not the traditional look at all, I call it the double decker stroller because one sits up and in front, and the other child sits down lower and back. You'll have to look it up to see pictures...it's pretty amazing and we get asked about it all the time. You can also buy the travel system (which is just a bar), which allows you to put the baby car seat on it. The other reason I liked it was because it's not as big and long as the traditional double stroller. My daugher is now 4 1/2, and almost 40 lbs. and is still able to sit comfortably in either seat when we go to Sea World, the Zoo, Disneyland, etc. Even when she's done riding in the stroller, we'll take off the back seat and use it as a single stroller until our son no longer needs a stroller. We feel we're getting our money's worth. There are two downsides...no cupholders (we attached some), and when you have both kids in it, you don't really have room for the diaper bag, because the basket is where the older child's feet go. We use a small backpack for a diaper bag, so that doesn't bother us so much.
Good luck with your decision...hope this helps!