I cannot directly answer your question but I can tell you my daughter's experience with a different sit-n-stand stroller that she used with her newborn and 3 yo children.
I looked at the 2 strollers you gave web addresses for and my daughter's babies' stroller is not either of those. I don't remember the brand of hers. This was 6 years ago. As I recall it had the usual stroller configuration in front and then a platform with a bench in back. The older child could stand on the platform facing forward or sit on the bench. The bench has a safety strap but I don't think there was anything to support the child while standing except for something to hang on to. The front position also held the car's infant seat.
The stroller is heavy and cumberson to put in the car trunk. We used it often taking the kids for walks to a park and the store and all of us liked it. I think my daugher used it less often when leaving in a car.
When we went to the mall my daughter used a light weight frame that held only the car's infant seat. I remember her saying that she preferred the small frame rather than a regular stroller because it was lightweight and easy to use. My granddaughter liked to help push the cart. She was 3 preferred to not ride in a stroller bur she did when there were 2 people walking. A 10-11 yo neighbor girl often went with us to push the stroller with my granddaughter in it or to hold her hand. I did use an umbrella stroller when I took her for long walks from my house. She walked until she got tired and then rode.
An aside comment: At 8 my granddaughter still likes to take walks. She and I walked often from the time she was a couple of months old. I didn't walk with my grandson and he doesn't like to take walks. May be my influence or not. :):)
Once my grandson had outgrown the car seat, my daughter and I used a light weight stroller with a net basket underneath the seat and on the back of the baby seat, a snack tray and beverage holder for the baby and the adult, and a canopy. It folded in a similar way the umbrella strollers fold. It was very easy to use; much easier than the regular stroller my daughter had for her first baby.
I think my daughter said that if she had to choose over again she'd choose to use the frame first and the lightweight that folded like an umbrella stroller for her oldest and skip the sit-n-stand. She was on a limited budget. However, those walks to the park would've been difficult without the sit-n-stand. The lightweight stroller wasn't so convenient for rough terrain.
The frame for the car seat was around $50 and the lightweight stroller was less than $100. She bought the sit-n-stand with a gift card.
Your oldest child will be much younger and this would make a difference in choices. I am impressed by the pictures of the Contours stroller. There wasn't a good picture of the double stroller but if it is just a double stroller I would choose the Contours one. There wasn't a good picture or description of the double stroller. I would want to know how the standing part is provided.
Your son will want to sit in front. At 3, my granddaughter did ride in a stroller when we had lots of walking to do such as at the mall. I think she's be less willing if she shared with brother. As a preschooler she often said, "I'm not a baby." Then there will be the dilemna of who sits in front with the 2 seater. Once your daughter is interested in looking she won't want to ride in the back. I've seen 2 seaters and the seats are awfully close together. What child wants to have their face 16 inches from a canvas back?
Your babies at 15 months apart will use a stroller together for at least a year and probably longer. I'd choose the contour because it has choices and isn't really very much more expensive.
I would also consider getting a car seat stroller frame for use in the first few months if there will be 2 of you to push 2 strollers or if one of you will be taking just the youngest. The frame is so lightweight and ergonomically designed; very easy to maneuver and relatively inexpensive. Not heavy enough for rough terrain but perfect for mall and in store shopping.
A comment about shopping. When I shopped for groceries with both kids, I put the car seat over the grocery cart child's seat and my granddaughter in the cart on a folded blanket. She could help put food in the cart and I always knew where she was. Shopping was faster and safer this way. Then when the youngest was able to sit in the child's seat I still kept my granddaughter in the grocery cart. When I just had my granddaughter she rode in the grocery cart that had a full sized plastic formed seat until her head was above the back. This was quite awhile.
As Sue suggested, I would look in resale shops. My grandchildren's strollers were in nearly new condition when they were thru using them. Your post reminds me that I was going to take them to a resale shop but they're still in my daughter's storage.
