T.,
I had two boys 20 months apart, and never needed a sit-stand, but my first was a good walker, good listener, and really easy toddler. If he hadn't been so good at walking with me in and out from ECFE and other places, it wouldn't have been so easy to do it with a single stroller. Also, I did have a two seated walking/running stroller which I carted around to the zoo and other places. I usually carried the baby once we were inside using a Bjorn or sling.
When I had my 3rd my older two were 3 and almost 2, and the sit-stand was a lifesaver for me. I used it EVERYDAY moving the 3 of them in and out from school for ECFE, the library, the mall, etc. The car seat clicked in the front, the 2 year old sat in the seat and the 3 year old stood on the standing spot. Then once we were inside, my oldest would usually walk. I bought it used from another Mama-Source mom who saw my post with the same question!
I'd think about what your schedule is like, how often you'll be carting kids in and out from places, the time of year, etc. Because my third was a winter baby, I had to move her in the car seat, so it was really nice to click it into the sit-stand instead of trying to carry the car seat and herd the other two where I needed them to go.
Good luck and congratulations!
Jessica
So, for me, with the 3 kids it was totally worth the $50 I paid for a used one. My Mom was going to buy me a new one as a new baby gift, but finding the used one was equally good in my opinion.