Hi R.~ Congrats on the upcoming birth of your second child. My two children are 2 yrs and 4 months apart so when we were expecting our second I chose to go with the sit-n-stand stroller as well. I went with the Baby Trend Deluxe Sit-n-Stand Stroller. It came with an extension bar that locked into the sides of the stroller to accomodate pretty much any infant carrier thats on the market. It of course also had the snack tray for when they are older & a parent tray on the handle bar. The infant seat securely latched right on and I didn't have to worry even when he was little. Its the same length as a double stroller, but gives your older child the option of riding while sitting or standing and is easy for them to quickly get off if they choose to walk for a little while. Its been a life saver when we have been at the malls, zoo and many other places. My daughter is very independent and loved the freedom of having choices. I know that Joovy makes a nice sit n stand as well. Its a little more expensive and a little more compact. (The handle bar on it is more straight up than on the Baby Trend...putting the older child more directly under you than w/ the BT in my opinion. The Joovy did not have a parent tray, both have a netted basket underneath for a diaper bag.) As others have said the downside to the sit n stand is that the older child will be facing you when sitting. This hasn't been an issue for my daughter b/c when she wants to see she either stands up or walks.
Take your time and try them out. I know that Babies R Us carried both types when we were shopping around. I'm not sure that they still do, but I also know that Target has started carrying the Baby Trend. After researching shop online to see if you can get a better deal!!! I got mine online from strollerdepot.com for about $40 less than it would have cost me at babies r us at the time. (that included shipping costs) I just happened to do a search & found the Baby Trend on Ebay new for $89, its $____@____.com sit n stand was and is the only stroller that I have used since my youngest was born. He's now almost 20 months. When my daughter, who will be 4 this week choses to use it she still has that option....and when we're shopping and she doesn't want to, its extra baggage space. :)
Also a note about double strollers. In my opinion if your going to go w/ a double stroller I wouldn't get a side by side. Before my youngest was born I was a nanny for another child the same age as my oldest and had a side by side double jogging stroller. I liked it, but it wasn't practical for shopping in some of the stores. Alot of stores such as Childrens Place, Gymboree, etc don't have wide enough isles to get the double side by sides through them. I sold mine when I bought my sit n stand.
Sorry for rambling on & on, but I wanted to try and give you as much info as I could from my personal experiences using both the sit n stand and the double that others have suggested. Hope that this helps in your search!