M.,
I have a Sit N Stand, which is fine. My 3-1/2 yo likes being able to get in and out, and she does pretty well so I don't mind her hopping in & out. If you do buy one, be aware that there are longer and shorter versions of the S&S. I have the shorter version, which we like. We use it when we are at the mall or the zoo, or something like that.
What we usually do, however, is put the toddler in a regular single stroller and carry the baby in a sling. My baby is so much happier being carried, and it is easier for me to maneuver a single stroller, as well as less space in the trunk. I make baby carriers, so I have a huge assortment, and I have my favorites for different needs. My most favorite carrier is my Mei Tai. I have been using it since DD was a couple months old. I didn't get to use it when she was a newborn because I lent it out to a friends and she didn't want to give it back, she used it all the time! They work great for newborns, though. I still carry her around in it all the time. She is 14 months and about 21 lb, and I carry her on my back. I also can carry her older sister in it quite comfortably for an hour or more. If you want more information about the different slings I make, or if you just want to chat about what kinds of slings are out there, feel free to email me. I love to talk baby carriers!
If you decide on a double, start checking out consignment stores and garage sales. We got a great deal on our S&S at a garage sale - $40! Target.com also has a really nice selection if you want to buy used. They have reviews of most products posted by mamas that use them, so if you are thinking of specific models of stroller you can read up on them and decide which sounds best.
Best of luck,
S.